There was a study I remember seeing referenced a while ago (I think it was in an article about the Mayo clinic guy Levine) where the big point was that if you spent essentially most of your workday sitting, then an hour at the gym doesn't undo that. With this approach I can walk 3 - 6 miles while working. I'm not trying to get my heart rate up to cardio levels obviously; I'm just walking. And I'm not saying I'm a role model for perfect health, etc...