This is Part 5 of our series on How to Write Your Own Training Plan. You’ll find Part 1 here. Over the next 9 posts we’ll cover the following topics:
1. Intro
2. Definitions and Terminology
3. Where You Are and Where You Want to Go
4. The Elements of a Training Cycle
5. Workout Purpose & Design - Endurance (⭐You are here.)
6. Workout Purpose & Design - Threshold
7. Workout Purpose & Design - Speed & Hills
8. Putting it All Together - Micro, Meso, & Macrocycles
9. The Extras - Strength, Cross-Training, & Course-Specific Add-Ons
10. Taper & Race Prep
The Science
The body’s aerobic capacity that allows us to run for more than a few minutes is built from a complex set of interrelated systems, all designed to efficiently supply oxygen to cells. While you don’t need to know the science of how those systems work and affect one another, I think it can be helpful to understand how different workouts apply stress and create adaptation to these mechanisms. With that goal in mind, let’s walk through a brief description of what happens each time you inhale.
Air fills the lungs, where receptors on red blood cells bind with oxygen (O2) molecules. The oxygenated blood is then pumped through a network of vessels and smaller capillaries, eventually reaching working muscle fibers. At the cell wall of the fiber, the oxygen is driven by a pressure differential across the cell membrane, where it is then bound to another transporter molecule that moves the O2 to the mitochondria. This is where the real action happens.
In the mitochondria, a series of electrochemical reactions is happening that move electrons from protein to protein like a baton in a relay race. The anchor of that race is oxygen, which runs the final leg, combining with hydrogen ions to form water and—importantly—carry the final electrons in the chain across the finish line. Moving the electrons is the real goal here, in a process that is analogous to recharging a rechargeable battery. The electrical charge stored in the battery can then be converted to ATP, the energy currency of cells.
98% of the oxygen you breathe is consumed by this process of cellular energy production. When you’re at rest, the required oxygen is delivered easily and the system can operate at a fraction of its total capacity. When your energy demands increase (as they do when you’re running), the body’s ability to deliver and utilize oxygen becomes much more critical. At easy pace, you’ll be using 60-70% of your total aerobic capacity, and by the time you get up to about 3K pace, you’re working at 100% capacity. The more oxygen your cells can consume, the greater your aerobic capacity, allowing you to produce more energy and in turn, convert it to speed and power. The measurement of your aerobic capacity is VO2max, which is measured in liters of oxygen per minute, and a higher number indicates a greater ability to produce energy through aerobic pathways.
Training Aerobic Capacity
In training the aerobic system, our goal is to find ways to increase our ability to supply oxygen and in turn, produce more energy. My reason for the brief foray into biology here is to demonstrate both the variety of mechanisms that can be affected by training and their interdependence:
In the quest to improve aerobic capacity, we would first want to increase the number of mitochondria within each cell to allow more parallel capacity to produce energy. In order to do that, we would need additional transporter molecules to move the oxygen within the cell, as well as more capillaries to deliver red blood cells efficiently to the fiber. While we’re on the subject of red blood cells, it would be good to have more of those too, and a higher blood volume in general. And oxygen-rich blood is only useful if we can move it around easily, so the heart also needs to be able to push more blood with each pump.
Each of these improvements can be affected by training, but the adaptations happen on differing time scales and are stimulated by different mechanisms. Understanding how various workouts stress each part of the system will inform your decisions about format, duration, and intensity, and can also give you a better sense of why you might respond better to some types of training than others.
Two Sides of the Same System
I tend to think of aerobic capacity as a pyramid with two key dimensions: there’s the size of the base, which determines things like how far and how long you can sustain an easy pace, as well as how fast your easy pace is. The other measurement is how tall your pyramid is, which is analogous to how much speed and power your aerobic system can produce when it’s working at 100% capacity. If you only ran easy miles and long runs, your pyramid would have a wide base, but not be very tall. You might be able to run for 3 hours comfortably, but would likely struggle with the intensity of a 5K. Likewise, if you only trained VO2max workouts but never ran more than 5 miles on your easy runs, you might run a solid 5K, but would not be optimized for longer distances, and your pyramid would look more like the Eiffel Tower.
Long Runs and Easy Miles
The proportions of your pyramid might change with your goal race distance, but as a general rule, the larger your base, the taller you can build your pyramid. Aerobic base training improves physical structures of the system, and in general, it’s accomplished through sustained accumulation of fatigue applied at a low intensity over long durations. Keeping your long run effort around 60-70% VO2max ensures that the aerobic system is the primary means of energy production.
The most oxidative slow-twitch fibers are the first to be recruited in any endurance effort, but part of the adaptive magic of the long run is that even the most efficient muscle fibers tire over time, forcing less aerobically-adapted fibers to share the load. Recall that hybrid and fast-twitch fibers are more reliant on glycogen for fuel, and as fatigue increases and their fuel supplies run low, signals go out to fortify these fibers with more oxidative capacity. Over time, your hybrid and FT fibers will build new mitochondria and increase capillary density to forestall future low-fuel situations. At a systems-level, the heart and diaphragm are also strengthened by long-duration efforts, becoming more efficient at delivering blood and oxygen and removing metabolic byproducts from working muscles.
Easy miles distributed throughout the week also contribute to aerobic adaptations. While you’re unlikely to drain fuel stores in a 6-mile easy run alone, you may still get some of the same signaling when glycogen stores are a little low after a speed session the previous day. Many of the other adaptation signals of the long run are also stimulated by the accumulated mileage (and fatigue) of the week. Just like the rest periods between hard intervals, easy runs are the recovery periods between hard workouts.
Endurance Workouts
The long run is the keystone workout for endurance training, and can be done by time or distance. They should be as continuous as possible. A periodic stop to refill a water bottle or wait for a traffic light is fine, but if you need to pause frequently to catch your breath and reset, you’re going too fast. In general, long runs need to increase in length and duration to elicit continued adaptation throughout the training cycle, though an increase in overall weekly volume can also contribute. Some coaches caution against running long runs that last more than three hours with the argument that they require too much recovery time. I see no reason to arbitrarily cap duration, provided that you properly fuel and hydrate to support the time and distance. As a coach, it seems far better to me to fully prepare a runner for the time and distance they’ll be racing than to send someone out for a race that could be 50% longer than their longest training run.
The longer your run, the more critical fueling and hydration become. Despite their reliance on fat as the primary fuel source, long runs still need supplemental fuel to achieve maximum benefit and recovery. As discussed, glycogen is still a necessary fuel source for some of the muscle you’re training. Taking in 25g carbohydrate every 30-40 minutes will allow your muscles to keep working, help maintain the quality of your workout, and ensure that your body isn’t too depleted to recover.
Other forms of endurance runs include doubles (two workouts in the same day) and back-to-backs (long runs two days in a row). Doubles and back-to-backs leverage accumulated fatigue to stimulate greater adaptation in the second workout than it would elicit on its own. Doubles can be a good way to increase mileage and aerobic load if they’re additive to your overall volume. Adding a 3-4 mile shakeout in the evening after a speed session in the morning will produce some of the same benefits of a long run since your muscles are starting in a fatigued state. Splitting an 8 mile easy run into two 4 milers is fine if that’s the only way you can get the miles in, but the two 4’s will not provide the same adaptations as a single 8. Likewise, back-to-backs are often utilized by ultrarunners to mitigate the stress and impracticality of doing a 40 or 50-mile long run by splitting the distance over two days.
You can also incorporate intensity into your long runs, and this is most commonly done by doing a specified mileage or duration at or around race pace. As with any added stress, the application needs to be thoughtful and judicious. Generally including 10-15% of your weekly mileage at half or marathon pace in a long run is a workable amount. Introducing threshold or faster intervals into a long run should be done carefully, and I highly recommend positioning those reps earlier in the run when your muscles are less fatigued. Progression and fast-finish runs are also a great way to include some faster paced miles in a controlled way. Because the intensity increases slowly, there’s less chance of injury, and they also train the ability to negative split and pick up the pace at the end of a long run. Examples of each are below:
Progression Long Run: 4 miles Easy | 6 mile progression from easy to MP | 6 miles Easy
Fast Finish Long Run: 12 miles Easy | 4 mile progression from easy to MP
VO2max Effort
So far we’ve talked a lot about how to build a strong aerobic base, which I’ve likened to the foundation of your fitness pyramid. Now we’ll discuss how to build speed endurance on top of that base, and how the two interact. We’ve established that long runs stress cells by applying a moderate stress to the aerobic system for an extended period of time, testing the system’s ability to manage fatigue over a long duration. VO2max workouts, on the other hand, apply a high stress for short durations, testing the system’s ability to work at maximum capacity.
Most runners can sustain VO2max pace for about 5-8 minutes all-out, putting it somewhere between 1-2 mile race pace. It’s a speed that is clearly much faster than lactate threshold, meaning that it will require energy production from both the aerobic and anaerobic systems. At these paces, more fast twitch muscle fibers will be enlisted to output the required power and speed, and they will produce their energy from glycogen stores. We’ll talk more about exactly what happens when energy production from anaerobic glycolysis overtakes aerobic processes when we discuss lactate threshold, but for now, here’s what you need to know:
When FT fibers are producing energy from glycogen stores, the anaerobic process used can only extract a fraction of the energy stored in the glycogen molecule. After the initial reaction, the lactate molecule that is left over can’t be utilized anaerobically, but the remaining energy it holds can be released in the presence of oxygen. As lactate is produced, it’s transported out of the working muscle fiber and into the bloodstream. This is where the aerobic system comes into play paces that are faster than LT. Slow twitch fibers may not be able to produce as much force and power as their FT counterparts, but their ability to process a wider variety of fuels allows them to receive circulating lactate and extract the remaining energy stores. At VO2max pace, the aerobic system is working at full capacity to support energy production from the anaerobic system. In other words, the larger your aerobic capacity, the more you can utilize your anaerobic power in endurance events.
VO2max Workouts
VO2max workouts are very intense and therefore need to be carefully structured. The effective dose is generally between 8 and 20 minutes, and workouts are done in an interval format. Rep length can vary, but most workout structures rely on short recovery periods so that stress accumulates over the course of the workout. Depending on the rep duration, it might take several repeats to actually reach VO2max intensity, and this should be considered when designing the workout. Work-to-rest ratios are usually equal for short intervals (less than 1 minute), and shift to 2:1 or 3:2 as the work interval increases. Because the recovery periods are relatively short and the aerobic system is under stress throughout, I count the entire workout during (excluding warm-up and cool-down) toward the week’s 20% quality.
Examples of VO2max workouts are as follows:
10-16x 30sec @ VO2max / 30sec Easy
6-10x 60sec @ VO2max / 60sec Easy
4-6x 2min @ VO2max / 1min Easy
2x
3min @ VO2max / 2 min Easy
2min @ VO2max / 90 sec Easy
1min @ VO2max / 45sec Easy
Putting it All Together
I hope I’ve convinced you that while putting in the miles is critical, endurance training is about more than long, slow, distance. If you want to develop not just endurance, but speed endurance, the ability to work efficiently across a range of paces and intensities is critical. Next week, we’ll delve into threshold workouts, and explore the role of lactate, not as a waste product, but as the pivot and link between our aerobic and anaerobic energy systems.
