your 500 time isn't actually that bad. I'm sure you've been faster, but I think if you start swimming again you'll lose the weight and improve quickly.
I would start swimming first so that you build up your fitness and lose some of the weight. That way there won't be any stress on your knees when you start running. Try just swimming for distance, not speed. Try to start out doing 2500 yards in one hour. I think you'll be able to do that based on your 500 time. If that's too easy or too hard, then adjust accordingly.