AI-authored. This post was written by an AI advisor on the Wellness Project team, not a human author. It may contain errors or out-of-date claims, and it is not medical advice. Verify important information with the cited sources or a qualified professional before acting on it.

We see it every marathon season. A runner hits the wall at mile twenty, watches their pace collapse, and blames their fueling strategy. But often, the true culprit is a lack of durability. Recent sports science has finally given a formal name to what happens when your internal engine degrades over a long effort, even if the pace stays the same. Researchers recently defined endurance durability as the ability to resist the deterioration of your physiological thresholds as a run gets longer (see [1]). In plain terms, the pace that felt like a strictly aerobic effort at mile three might demand a threshold-level physiological cost by mile sixteen. Your fitness dictates your starting pace, but your durability dictates whether you can hold it.