Gabe:
You should probably be training your legs. After your stress fracture heals of course. What you should focus on is strengthening the muscles in your legs so that you can cushion the impact of the stress you put on it, as well as bone will reshape and strengthen itself to the given stresses put upon it such as increased muscle mass. So your bones will actually be thicker and denser as you gain muscle. This doesn't mean run and stuff though. I actually mean go to the gym and do some strength training. Squats, lunges, those sorts of things. I believe the reason why you injure yourself is because your body is unaccustomed to all the physical strain you have been putting on it as of late. You go from a very sedentary lifestyle to an instantly active one, especially with the TKD club activities you've been telling me about and from what I remember the activities deal with alot of impact on the legs, which is okay if your body is adjusted to it, but not so great if it isn't. Especially if you're doing these things often and you're not allowing your body enough time to recover.
So like I said, think about doing some leg training. Lunges, Squats, Deadlifts are all good. However the thing is that they're also all technical lifts, so you might want to get someone you trust who know how to do the exercises to show you how to do them properly. You can also help improve your leg strength by doing things like skipping, or jump training like box jumps and stuff. What I'd reccommend is doing it on a somewhat thick mat, and focus on landing in the power position. What a mat will do is assist in absorbing the impact of the jumps so it's a bit easier on the knees and the feet as well as making the jumps harder because you have to put more effort into each jump. I can't really think of anything else, but there shouldn't really be a need to do alot really. Just stick to the basics and focus on form and technique as well as remembering to properly stretch and give your legs time to recover. That will be the best way to improving your hardiness. I can think of some other exercises but they're the more advance ones that I use to vertical jump training and are not very fun.
You might also want to check your shoes. It may have to do with orthopedics as well, especially if you're not distributing weight equally throughout your feet, that can also cause unnecessary strain on certain parts of the body.