My exercise of choice is running. The picture to the right is of me crossing the finish line of a YMCA-sponsored 5K that I did in the Rochester, New York area. I love to run, but recently I've had to walk due to some problems with my thyroid. I have hypothyroidism and I'm in the midst of getting my medication to the right dosage so that I have the energy to run again. It really sucks, not having the energy to run, but I know I'll get back there.
Running, for me, is not only mentally and physically healthy, but a chance to think about my writing. When I run, I get into a kind of meditative state. In this state, ideas often come to me, or I work though problematic ideas. It's pretty awesome. I can do this when I walk, too, or engage in other exercise (swimming, for example, which I really enjoy). I think it's running that gets me into the most meditative state, however.
I'm not suggesting that every writer who reads this take up running. It's not for everybody. Some people just plain hate it, while others may have physical issues which prevent them from being able to run. However, I do suggest taking up some form of exercise if you don't exercise already. It's amazing how much it can both clear your head and fill it with ideas at the same time. And the endorphin rush you get at the end of a good workout is amazing. It makes me want to write and create and be productive in any capacity.