You’re probably asking yourself, “Why would he think I’d care?” Well, you’d better care. Stress of one kind or another is the number-one cause of relapse.
Phys Ed: Why Exercise Makes You Less Anxious – Well Blog – NYTimes.com

For years, both in popular imagination and in scientific circles, it has been a given that exercise enhances mood. But how exercise, a physiological activity, might directly affect mood and anxiety — psychological states — was unclear. Now, thanks in no small part to improved research techniques and a growing understanding of the biochemistry and the genetics of thought itself, scientists are beginning to tease out how exercise remodels the brain, making it more resistant to stress.