I walk 3x a day for 30 minutes and it works for me. I have 4 kids and I feel guilty if I'm gone for longer than 30 minutes. I put on the CD player and go.
I tore ligaments in my knee years ago and can't walk for long periods of time but if I rest it for 3 or 4 hours it's enough time to go out again.
I just love the fresh air listening to good music it's very rejuvenating.