I get your point about walking but running? Every personal reason to be running outdoors ultimately ties back to the exertion involved. For example, running to "clear one's mind" is predicated on the effect physical exertion, cardio exercise in particular, has in calming/resetting/clarifying one's state of mind. I guess I could have clarified my original point better as I meant it more in terms of someone choosing to derive personal benefits from physical exertion.