The power of a second wind
This guy goes out for a run and he reaches a point where he's getting fatigued and short of breath, but he's far short of his planned destination, so he keeps on trucking. Then as he goes along, at some point he realizes he's not puffing anymore and he has more energy than he did earlier in the run. This happens. It has happened to me. What has happened is that the runner has got a "second wind."