Swimming also puts you in better shape IMO. I used to run plenty, but my knees started hurting, so now I only run if I am actually playing a sport. I took up swimming for my exercise. I'd never taken a swimming lesson in my life, but I just went to the pool and started doing it. A few times I asked the life guards for some tips, and I also chatted up some of the good swimmers and got tips from them.

I went for a 2k run the other day just to see what it was like after swimming but not running for a couple months. It was about the same as when I ran everyday (maybe even a bit easier), and I have a slightly more toned upper body now.