It’s one of the questions most people ask, especially those who are about to debut in a marathon. Does doing long runs of up to 30 km really prepare you better for the marathon? The key isn’t only how far you can run, but how well you can sustain your performance when fatigue sets in.
1. Durability matters.
A study involving 82,303 marathoners observed that the decoupling between heart rate and speed began, on average, around the 25th kilometer.
Runners with lower decoupling experienced it later (33 km compared to 19 km in those with higher decoupling) and achieved faster times. In other words, it’s not only your initial aerobic capacity that matters, but how well you manage to preserve it during the effort.
2. Accumulated volume also counts.
A meta-analysis of 85 studies found associations between better performances and higher weekly volume, training frequency, and length of runs. But association does not imply causation: it does not prove that reaching a specific distance is essential.
3. What happens when we prolong a session for too long?
Exertion increases on different systems:
→ Metabolic: depletion of glycogen stores.
→ Neuromuscular: fatigue and possible deterioration of running economy.
→ Musculoskeletal: accumulation of loading cycles on bones, muscles, and tendons.
→ Recovery: a greater load may require more time to assimilate.
There is no universal physiological threshold beyond which a run ceases to be beneficial.
The question we should ask ourselves is not how long a runner can endure, but what stimulus they need and what recovery cost it entails. Because a good session is not necessarily the one that generates the most fatigue, but the one that contributes to improvement without compromising subsequent training.
