Writing and Exercise: Similar?

In order to avoid betraying my utter ignorance, I’ll speak in vague generalities: if you’re walking or running efficiently, your body works a bit like a pendulum, swinging (not forcing) itself forward. I feel that I could run or walk for hours and never get tired when I’m running or walking in that ‘groove’. One long hill later and I have a different idea, but let’s not focus on that right now.