I wouldn't say joint problem. Extensive training for these kind of endurance races stretches the ligaments and the cartilage within the legs. If anything, I'd say it would probably be ligament damage. Nothing that a bit of rest for you leg won't fix. Thats my diagnosis. However, I'm not a doctor. Nor are many people on here, so it may not actually be that at all. However, I have studied biology quite a lot and thats what i'd go with.
Anyways, well done on finishing the race. Might do it next year myself. So if you're gonna do it again, I may well join you. I've always wanted to do it so I might as well sign in for it.