There are all sorts of explanations, and as the article points out, there weren't enough controls to sort among any of them. Intellectual stimulation? Sunlight (which has significant physiological and emotional effects)? Body mechanics? Fresh air (I know I breathe differently outside than indoors - can't say why or how, but it sure feels different)?

Even on a track you have some lateral movement to avoid obstacles, and the ability to change speed quickly to react to obstacles, start an interval, etc.