I work at a very sedentary desk job. Although I used to be very athletic and active, a bad back injury in '95 turned me into a couch potato. Recreational athletics were replaced with recreational eating. Here I am in April of 2010, 48yo at 220lbs.
At the end of 2009, I took a new position that no longer required me to drive all over town most days, so I started riding my bike to work as much as possible. Here I am one year later, in June of 2011, 49yo at 162lbs.
I also changed my eating habits quite a bit. I'd highly recommend Gary Taube's Why We Get Fat and What To Do About It. It was actually pretty easy.