You feel you are all set for the marathon. You have trained your mind & body by running tons of kilometers at your goal pace. On the race day, you start your run and you check your watch, the heart rate is low, the paces are coming, and you feel invincible. But fast forward to the thirty second kilometer - ten more kilometers to go. You’re consistent with your pace, yet, that same pace now feels tough. Your heart rate is drifting upward, your breathing is labored, and maintaining rhythm feels like a challenge. We have accepted this, and we call it “fatigue” or “hitting the wall.”
New physiological research suggests something more specific is happening here. The runner you are at the finish line is physiologically not the same runner you were at the start. This concept is called Durability.
Traditionally, running fitness has been defined by three physiological pillars, often measured in a lab when an athlete is fresh:
As we train for marathons, we focus on improving these three numbers. However, we assume these numbers are static for a given race. For example, if Lactate Threshold is a 5:30/km pace, we assume that number stays static for the whole race. But in reality, it doesn’t.
Recent research suggests that these markers are not stable. As you run for 90 minutes or more, your physiological profile deteriorates. Your “fresh” stats decay into “fatigued” stats.
Here is what is actually happening as we go long into a long run:
1. Your Threshold Moves Down
This is the most critical factor. When you are fresh, your lactate threshold might be high. But after 90 minutes of running, that threshold drops significantly.
The result is that a pace that was comfortably “aerobic” at the start of the race might drift into the “anaerobic” zone by the end, even if you don’t run any faster. You aren’t just tired; you are physically crossing a metabolic threshold because the threshold moved.
2. Your VO2 Peak Drops
Your maximum capacity to process oxygen actually shrinks. Studies show that after prolonged running, your VO2 Peak can drop by roughly 6% to 7%. The result is that you have less total horsepower available to surge up hills or kick to the finish.
3. Economy (Usually) Holds Steady
Interestingly, based on the research for well-trained runners, running form (economy) doesn’t always fall apart. The breakdown isn’t necessarily mechanical; it is metabolic. You aren’t necessarily running “sloppier,” you are just running with a less efficient internal engine.
This brings us to Durability. Durability is defined as the resilience of your physiological traits over time. It is not about how fast you are when you are fresh; it is about how much speed you lose when you are tired.
The data shows that Durability is highly correlated with marathon performance.
In a field of runners with similar VO2 Max and similar fresh speed, the runners who slow down the least are the ones whose internal physiology deteriorates the slowest. The runner who keeps their Lactate Threshold stable at 35th kilometer beats the runner whose Threshold plummets.
1. Practice “Fatigued Running”
Many runners do their speed work on fresh legs (e.g., Tuesday intervals). To build durability, you need to run fast when you are already tired.
Add tempo blocks to the end of your long run. For example, run 12 miles easy, then pick up the pace to marathon goal pace for the final 3–4 miles.
2. Master Your Fueling
The deterioration of your threshold is closely linked to glycogen depletion (running out of carbs). When glycogen gets low, your body shifts to burning fat, which requires more oxygen to produce the same speed.
Train your gut. Practice taking gels and fluids during training so you can keep your carbohydrate stores topped up during the race, delaying the physiological drop-off.
3. Be Conservative with “Fresh” Data
If you just ran a 5K PB, don’t use a generic calculator to predict your marathon time. That 5K reflects your fresh physiology. A marathon requires your durable physiology. Be slightly conservative with your start pace to account for the inevitable drift in your threshold.
This article is based on recent findings regarding physiological durability in endurance athletes, specifically the study:
Hunter, B., & Muniz-Pumares, D. (2025). Durability of Parameters Associated With Endurance Running in Marathoners. European Journal of Sport Science. (This study tracked London Marathoners and found that the durability of their Lactate Threshold was a key predictor of race performance).
Compiled by Team GeeksOnFeet for the love of running!