Just a quick note on "toning" your muscles... that's sort of a misleading concept. Basically, you can build muscles, and you can lose fat. One has to do with how much weight-bearing activity you do, and the other has to do with the number of calories you burn in a day exceeding the number of calories you consume. Neither of these things is something you have uber-control over, the way most things that offer to "tone" you would lead you to think. You can't just lose fat in a targeted area of your body - you lose it all over, and it generally goes away in certain places first thanks to your specific genetics (ie. you can't change where your body wants to drop its fat first).
That said, I think you should mainly focus on doing cardio. Swimming would probably be ideal, since it's so easy on the joints and works out a ton of different muscles. I personally would lift some weights, too, but I just happen to think very highly of weight training. I found
this site to be a very useful source of good information and encouragement on that front, if you're interested.