I work out at a local gym about five times a week.
It keeps me somewhat sane and I know it has been invaluable to me as a writer.
I used to be a real couch potato until I realized in my mid-twenties that gym life was actually kind of cool. I don't take classes because I'm too stubborn and a bit of a non-conformist ("No one tells me when to kick! I'll kick when I like!"), but I love burning calories by working out (in a self-directed way) on machines. Afterwards, I pig out and don't feel too guilty about it.
I truly wish more literary artists took the time to care for their bodies as well as they care for their creations. I know many authors who lament that they sit all day; would that they could allow themselves the pleasure of daily walking or jogging breaks.
Not only does regular aerobic exercise increase your stamina (so you can handle a few nights of writing until 2:00 a.m. and then getting up five short hours later ), it also provides your mind with "free association" time. I often mentally "escape" during physical activity and when I return to my computer, I'm much more relaxed, inspired and refreshed.
And there's another upshot to staying healthy, especially for those of us who routinely hear some variation of: "No, your work isn't good enough." When you're super-duper-pooper-scooper pissed off, running and/or lifting weights gets out your aggression in a productive manner that you're not likely to regret.
If you aren't accustomed to treating yourself to the luxury of a little exercise, I invite you to start now.
Even if you hate the concept of "formally" working out, you probably like some type of heart-pumping activity, whether it's hiking, cycling or even kissing. (I read once that "heavy petting" sessions can burn calories... what an awesome excuse for some late-night necking! Back seat sex optional.)