Pacing for Ultra Events: How Durability Translates to Race Day
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Most pacing advice is built for races that finish before your physiology starts falling apart. A 5K finishes before substrate shift matters. A marathon finishes around the time durability becomes the dominant variable. An ultra starts where the marathon ends.
That's why pacing strategy for ultra running is its own discipline. The numbers from your last threshold test, the heart rate zones on your watch, the splits you ran in training: at some point they no longer apply as the physiology shifts underneath.
This post walks through what changes during a long event, why durability matters more than fresh fitness on race day, and the practical pacing rules that follow.
What "Durability" Actually Means at the Start Line
Durability is how well your physiological thresholds and work capacity hold up during prolonged exercise. The simple version: how well does your fitness hold up when you've been working for hours.
In trained cyclists, the Rothschild 2025 study showed that after about 2.5 hours of riding at moderate intensity, threshold power had fallen by an average of 10 percent across 51 athletes. Some dropped 1 watt. Some dropped 45.
In trail runners, the same thing happens. After 180 minutes of submaximal running, 23 trained trail runners ran an uphill time trial and went 6.6 percent slower than their fresh-state version of the same effort. Their oxygen uptake was preserved. Their heart rates were near max. The mechanical output had simply gone down.
The point is not that fresh-state fitness doesn't matter. The point is that on race day, you don't run the fresh version. You run the version that exists at hour five, hour eight, hour twenty. Your pacing has to be built around that version, not the lab one.
Why Tour Mountain Stages Are a Durability Lesson
On a hard mountain stage, twenty riders crest the first climb together. Ten make it over the second. Five make it over the third. The riders who get dropped do not have lower VO2max than the riders who stay on. Their fresh thresholds, on paper, look the same.
What separates them is durability. The riders who last are the ones whose fitness held up under hours of preceding work. The riders who dropped had similar fresh fitness but couldn't carry it through.
This is exactly the variable an ultra rewards. A 100K, a 100 miler, a backyard ultra: these events are not won by the athlete with the best fresh threshold. They are won by the athlete whose threshold is still close to fresh at hour eight, ten, twenty. Pacing for these events means racing the durability version of yourself, not the lab version.
The Fixed Heart Rate Ceiling Problem
The most common pacing mistake in long events is treating heart rate zones as fixed.
Two things happen at the same time during a long effort. Your heart rate at any given pace drifts upward. We covered this in cardiac drift explained. And your threshold drifts downward. After 2.5 hours of cycling, Rothschild et al measured VT1 before and after found heart rate was 151 bpm versus 142 bpm fresh, power dropped 21W on average. The threshold has fallen, and heart rate at that threshold has risen.
That means a fixed heart rate ceiling, the popular "stay below 135 bpm" rule, fails in two compounding ways during a long race. Some of the heart rate drift is benign, just plasma volume and temperature. If you slow down to keep heart rate flat, you're throwing away pace you could be holding. Some of the drift reflects a real threshold drop. If you keep pushing the same pace through it, you're now training above threshold without realizing it.
Maunder's applied take is direct: An athlete using a fixed heart rate ceiling on a long event will likely be undertraining late in the session. But if they don't have any other signal to confirm that, they're flying blind.
The practical answer for ultras is to stop pacing on a single fixed heart rate number after the first hour or two, and to layer in other signals.
Pacing Rules for Ultras
A few principles follow from the science:
Start slower than you think. Almost every pacing study of ultra events finds the same thing: athletes who run the back half close to the front half tend to outperform athletes who run the front fast. The cost of starting too fast in an ultra is not the early effort. It is the accelerated durability erosion that follows. Going out two percent too fast can cost ten percent off your back half.
Pace by effort more than by heart rate. Rate of perceived exertion (RPE) is the most reliable single signal you have during a long event. Maunder's view, supported by the breathing rate research, is that perceived effort tracks something real about how much central drive you are using to sustain the work. Heart rate alone misses this. Effort doesn't.
Anchor to breathing, not just heart rate. Breathing rate rises with central command and effort, not with metabolic demand directly. As your durability erodes, breathing rate at the same pace drifts upward. That signal is much stickier and harder to fake than heart rate, and it tells you something heart rate cannot. We covered the mechanism in why breathing is the missing link in endurance training.
Fuel proactively, not reactively. If you wait until you feel low to take in carbohydrate, you are already behind. In a study on trail runners, they took 90 grams per hour and still saw fat oxidation rise and carbohydrate oxidation fall. Carbohydrate during a race does not eliminate the substrate shift, but it slows it down meaningfully and protects the late-race threshold.
Don't fight cardiac drift in the heat. A hot day will produce a much larger heart rate drift for the same metabolic cost. Discount summer drift heavily before drawing pacing conclusions from it. The drift is mostly cardiovascular, not metabolic, and panicking and slowing too much is a common heat-day pacing error.
How to Read These Signals on Race Day
Tymewear's VitalPro chest strap captures heart rate, minute ventilation, and breathing rate at the same time. On a long race, the combination is what tells you which way the drift is going. If heart rate is up but ventilation is flat, the drift is mostly cardiovascular and you can hold pace. If ventilation and breathing rate are both rising at the same workload, the work has actually gotten more expensive and it's time to back off.
Where to Go Next
For the wider framework that ultra pacing sits inside, see the cornerstone on durability training.
For the heart rate side of pacing specifically, see cardiac drift explained.
For the substrate side, see fat oxidation and durability.
For the broader threshold framework, see the cornerstone on VT1, VT2, and VO2max. For the marathon-specific pacing application, see the intermediate marathon training plan.
To measure your own pacing signals across a long race, see the Tymewear VitalPro chest strap.
References
- Maunder E, Seiler S, Mildenhall MJ, Kilding AE, Plews DJ. The importance of durability in the physiological profiling of endurance athletes. Sports Medicine (2021).
- Rothschild JA, Gallo G, Hamilton K, Stevenson JD, Dudley-Rode H, Charoensap T, Plews DJ, Kilding AE, Maunder E. Durability of the moderate to heavy intensity transition can be predicted using readily available markers of physiological decoupling. European Journal of Applied Physiology 125:2911 to 2920 (2025).
- Spragg J, Leo P, Swart J. The relationship between physiological characteristics and durability in male professional cyclists. Medicine and Science in Sports and Exercise 55(1):133 to 140 (2023).
- Coop J, Maunder E. Koopcast Episode 160: Durability in Ultrarunning.
- Tymewear vs Cosmed K5 paired session validation analysis (internal, 2026).