Here's what you do.  Train for and run a half-marathon.  Then, whenever you start feeling guilty about not exercising or eating like a pregnant lady, just remind yourself of that day you ran 13.1 miles all at once.  Pat yourself on the back (do this figuratively; literally might require exercise) and eat another cookie.  So far, I've ridden this wave of rationalization for 6 1/2 weeks.  One day, I'll get new shoes and get back on the horse.  One day.