Isn't a standard track only ~a quarter mile (400m)? With a 40 lb pack, twice a day, plus hills, on young bones, with poor judgment about balance and weight placement, someone's going to get hurt. Stress fractures didn't seem implausible, though I suppose ligament injuries are more likely?

And yes, every specialist I've been to has said all I can do is try to protect my joints until I eventually decide I want surgery.