Five factors that decide how fast you can run

Two runners with the same oxygen uptake can run 10K in 38 and 45 minutes. That is not unusual, and it is the whole point of this piece. There is no single number that says how fast you can run for a long time. There are five, and each explains a different part of the answer.

Physiologists have used three of them since the 1970s. Michael Joyner and Edward Coyle summed them up in 2008 in a paper that is still the starting point: how much oxygen your body can use, how large a share of that you can hold for a long time, and how much oxygen each kilometre costs. The last two are newer and come out of research on what happens when a run gets genuinely long, and on what speed above threshold contributes.

VO2max: the ceiling

VO2max is the largest amount of oxygen your body can use per minute, counted per kilo of body weight. It is the ceiling on how much aerobic energy you can produce, and it limits everything else.

The ceiling responds well to training in the first years and then less and less. A runner who has trained in a structured way for ten years rarely moves their VO2max by more than a few units, and most of the last minutes in a marathon do not come from there. It is still the factor watches and apps talk about most, because it is the easiest to estimate: one timed race is enough for a reasonable guess.

In practice it is estimated from races and hard efforts. Jack Daniels' tables translate a race time into a value he calls VDOT, which is not the same as a measured oxygen uptake but works well as a comparison over time. If you have not raced in a long while, no app has anything to go on, whatever it shows.

Threshold: how much of the ceiling you can hold

Threshold is the pace at which your body stops keeping up with clearing what it produces. Below it you can keep going for a long time. Above it the clock is counting down. For most recreational runners it sits roughly at the pace they could hold for an hour.

This factor explains most of the gap between the two runners in the first paragraph. Two people with the same ceiling can use a different share of it before they have to slow down, and that share can be trained for life. That is why threshold sessions are the backbone of nearly every half marathon and marathon programme.

Threshold comes in several versions, lactate threshold, critical speed, ventilatory threshold, and they are measured in different ways. Oliver Faude and colleagues reviewed the concepts in 2009 and why they do not always point to the same pace. From a watch it is estimated from threshold sessions and races: the pace and the heart rate in the work parts, compared across weeks. A single good session says little. Three similar sessions in similar conditions say something.

Running economy: what each kilometre costs

Running economy is how much oxygen you need to hold a given pace. Two runners with the same ceiling and the same threshold can still differ here, and the one who needs less oxygen per kilometre runs faster on the same engine.

It is measured with breathing gases in a lab, at a set pace, and it is the one factor no watch can measure. What can be followed is a proxy: pace at a given heart rate on the even parts of easy runs, adjusted for hills and heat. If you run faster at the same heart rate than three months ago, something has improved. But heart rate and pace cannot separate better economy from a higher VO2max, and heat, fatigue and measurement error move the relationship around. Andrew Barnes and Andrew Kilding describe in their 2015 review both how economy is measured and what improves it, and strength training and plyometrics are among the things that work.

So everything a watch or an app shows you about your running economy is an estimate, however certain the number looks. Coach Fartlek shows an estimated oxygen cost and calls it that.

Durability: what happens after 90 minutes

The three classic measures are taken at rest, on a fresh body. A marathon is not run that way. Two runners who are equal on all three can lose different amounts after two hours, and that loss does not show in a short test.

Ed Maunder and colleagues argued in 2021 that this is a factor of its own and called it durability. Andrew Jones calls it the fourth dimension of performance. It is the factor that decides the last miles of a marathon and explains why so many runners who "should" break 3:30 on their half marathon times hit the wall anyway.

The simplest way to follow it outside a lab is heart rate drift: how the relationship between pace and heart rate changes during a long easy run. Compare minutes 10 to 30 with minutes 70 to 90. If you get less pace per heartbeat toward the end, the drift is positive, and lower drift in comparable conditions suggests you hold up better. Heat and intensity affect the number, so one run is not enough. The studies in this area, among them Nuuttila and colleagues in 2024 and Hunter and colleagues in 2025, measure more than heart rate drift and do not validate the drift percentage as an exact measure. It is a proxy, like most of these numbers.

Anaerobic capacity: speed above threshold

The fifth factor is what you have above threshold. It decides the sprint, the hills and how much margin you have in threshold sessions. Sandford and colleagues reviewed in 2021 how the gap between your top speed and your aerobic speed, the anaerobic speed reserve, is used in training.

For a marathon it contributes little. For a 5K or a 10K it contributes more than many think, and for the runner who never runs fast at all, this is often where the missing speed sits. It shows in your data as the fastest continuous pace you have held for 60 seconds in recent weeks. If you have not run fast there is nothing to measure, and the curve underestimates you. A short fast stretch also cannot show how large the capacity is, only that it exists. Duffield and colleagues showed in 2005 that even 400 and 800 metres use both energy systems, and ordinary pace data cannot separate the contributions.

How to see them in your own data

None of the five needs a lab to be followed, but all of them need the right kind of run. VO2max needs a race or a timed test. Threshold needs threshold sessions with heart rate. Running economy needs easy runs at an even pace. Durability needs long runs over 90 minutes with heart rate. Anaerobic capacity needs you to run fast at some point. If the session type is missing, the number is missing, and an app that shows a value anyway has guessed.

Coach Fartlek follows the five this way, from the runs that sync from your watch via Intervals.icu, and shows next to every number how much recent, comparable data sits behind it: limited, moderate or strong evidence. A factor without runs is shown as not measured. What it looks like in the app, and what the coach does with the numbers, is in the post on 3.0.

Which one is holding you back?

That is the interesting question, and the numbers alone cannot answer it. The same five figures mean different things for someone racing a 5K and someone racing a marathon. A runner with three training days a week often has a limit in everyday life rather than in physiology, and then it does not help to know that durability is weak if the long runs do not fit anyway.

What usually holds: if you have not raced in a long time, your ceiling is unknown, not low. If you run every session at the same pace, threshold is probably what is standing still, and the most common reason for not getting faster sits there. If a marathon goes well to 30 km and then does not, it is durability, not the ceiling. And if you have no sprint and no hills, it is no surprise if you never run fast in training.

In Coach Fartlek the coach makes that call in the deep analysis, against your goal race and your everyday life, and marks each factor Maintain or Develop. What it costs is on the pricing page. But the reasoning above works just as well with a pen and a training log. The point is not the tool. The point is to stop looking for one number and start asking which of the five is yours.

Sources

• Joyner & Coyle (2008): Endurance exercise performance: the physiology of champions
• Faude et al. (2009): Lactate threshold concepts
• Barnes & Kilding (2015): Running economy: measurement, norms, and determining factors
• Maunder et al. (2021): The importance of durability in the physiological profiling of endurance athletes
• Jones (2024): The fourth dimension: physiological resilience as an independent determinant of endurance exercise performance
• Sandford et al. (2021): Anaerobic speed reserve
• Duffield et al. (2005): Energy system contribution to 400-metre and 800-metre track running
• Nuuttila et al. (2024): Durability in recreational runners
• Hunter et al. (2025): Durability in marathoners