Hi See Saw,
I do the same thing when I walk, or even take it one step farther. I'll play in my head one of the things that I'm working on. I wonder other people notice that I sometimes mumble as I walk. They'll think I'm crazier than I really am.
I think it's the forward motion of the rhythm that keeps us moving because we force our selves to keep up with the beat. When I play the piano, I find I play my best if I count out loud like I did whent I was first learning. If I count internally, I lose the beat and everything becomes unsteady.
It's totally weird how this works, but I like it.
John