Back on land, you will put less stress on your knees and hips by exercising on a stationary bike or elliptical machine. If you choose the bike, be sure your seat is well positioned. Also, don't ride without any tension on the pedals. You should have enough resistance so that the pedals quickly stop moving if you take your feet off them.
Yoga or Pilates would be a very different path to physical fitness. At first, you likely won't get as much aerobic training as you would from the other exercises mentioned above. But once you get familiar with the moves, you can.
If running is your passion, you will put less stress on your knees and hips by running on a real track rather than a treadmill.
Find out more about this book:Harvard Medical School Exercise: A program you can live with