If you can’t stand the thought of running, or just want to work out without a ton of pounding on your joints, do a few laps in the pool. It’s a low-impact exercise that will work all of your major muscle groups. As with most workouts, it helps to go in with a plan.
Try this one, from Rosante: Tread water for as long as possible by standing upright in the deep end and using your arms and legs to stay afloat. Then rest for two minutes. Now swim 10 sets of 100 meters (that’s back-and-forth lap in an Olympic-sized pool), resting for one minute in between sets. By the time you climb out of the pool, your muscles will be pleasantly worn out.