It's strange. Go out in freezing weather and stand still;you'll get cold. Move about and you get warm. I find it unlikely that you can heat up without energy?

The article mentions a study that tracked energy expenditure of runners over many weeks, and found that the energy expenditure was much lower at the tail end of the experiment, suggesting the body gets accustomed to the activity somehow and burns less energy.

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.

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.

Well one obvious adaptation is losing fat = less energy needed to move the body.

We saw a very large reduction in caloric expenditure in people who were already very fit runners. So body weight changes over the course of the experiment are unlikely to explain it.

So if they just kept doing the experiment they would end up not needing to eat?

Clearly there are some details that matter here...

And in most places in the article calorie expenditure is taken as "corrected for no-fat weight".