[uberduper]

The study must have tracked non-runners, given the conclusion. If they were already runners and the conclusion is that they get accustomed to the activity, they'd already be accustomed and there'd be no tail end difference. So they most likely tested non-runners and had them start running. We get more efficient at physical exertion with practice. Better coordination of muscle groups. Finding a more efficient pace. Correcting form deficiencies. These all increase capability or reduce energy expenditure.

[csee]

This assumes that there isn't a spectrum of running intensity. In this case they were running 10s of kilometres every day during the study, an absurd distance. So they were likely experienced runners already who were engaging in this extreme competition and took it up to an abnormal intensity that they weren't used to.