My first instinct would be to recommend cycling. Have you got a bike? The beauty of cycling as an exercise is that it at least feels productive - you've gone someplace and back with your bike at the end of your workout. You can take it at any pace you want, and you can vary the challenge by using a flat route or a hilly route. You mention your good leg strength, so this could be a perfect choice. A variation on this would be to try a spin class at a local gym - it's a whole workout routine based on stationary bikes.
Something else you could consider is trying something like tennis - you could join a class and get exercise that way whilst having fun and learning a new sport.
Just a couple of ideas, but you can run with those themes. No pun intended.
Wrestling. They have weight classes so you'll go against people in your weight. It will get you in so much shape and fit. Also if you start right now in middle school by the time you get in high school you'll have a ton of experience and will beat kids like nothing and possible get a championship. So TAKE A SHOT at wrestling (pun intended). The only equipment you really need is shoes, headgear and singlet. Don't mind the singlet they're really comfortable. Also you have good leg strength meaning you'll be able to take kids down with doubles.
Cycling its easy to start off I can't reli make an idiot of urself unless u fall and u will get in shape fast and no one will no ur doing it to stay on shape u can just cycle round ur area whereas with running its pretty obvious
Horse Back Riding! Best sport in the world I swear there really is nothing better in the world than riding horses and strong legs are a necessity for riding!
Try golf. Golf is not very physical :)
