I've switched from running to swimming. A confluence of factors conspired against the daily pounding I'd gotten into the habit of delivering to my knees and other joints. The main reason I indulged in this masochism was the excellent shape it left most of my body in (everything that is except my knees). Or rather, the excellent shape it made me THINK I was in. Today after only one lap in the pool I was totally dizzy and my heart felt like it was trying to pound its way out of my chest. I was too tired to feel embarrased that everyone kept cruising past me (even the old people with me in the slow lane). I knew I probably looked ridiculous but I was simply too exhausted to care. This is how it should be though, and secretly I was happy. I've discovered the perfect exercise, requiring both strength and finesse, and right now I'm terrible at it, so I can only get better.