I'm hanging up the shoes because my knees won't let me run pain free anymore. Although I can't really swim that well, I think it's the closest sport to running in terms of accessibility, meditative nature of it, and getting in a good aerobic workout with minimal time. First off, my technique is really bad and I can't swim the length of a pool without inhaling water. I'd probably have to take lessons. I took classes as a kid but failed so many times that I gave up. Are there any runners to swimmers out there? Do you enjoy swimming and much as you did when you ran?
I would take up cycling but I did a ton of that in college and it takes up too much time to get the same aerobic benefit as running.