One word: spinning. I just got back from a spinning class, which is like riding a stationary bike on crack in a sweaty room full of people for an hour. And it was amazing.
I was inspired by David and Kash's devotion to running, and decided to get in shape this summer. There's a certain masochism involved in getting in shape; it's hell while you're doing it, but produces a certain clarity afterwards. If you want to get in shape quickly, take a couple of spinning classes.