I am a huge fan of Crossfit. I began several weeks ago, and cannot remember being in as good shape since my peak in college (I was on a top-3 Division 1 crew team).

Be sure to follow the site's advice (and brianm's), however, about starting slowly. It helps to scale back some of the workouts to even a fraction of the time/intensity, and only working 1 or 2 of the workout sets (it is normally 3 days on, 1 day off).

My advice: start small, build up, don't give up!