In reply to Indy:
Not ideal for you of course, but I found XC skiing a great, non-impact cross-training exercise this winter.
As was mentioned, if you're already doing the running, complement with plyometrics/body weight exercises to build core strength. Pilates?
I don't have a gym nearby, and its still to snowy for biking (but not enough for XC skiing) so do a few pull ups on a bar at home, and core stength exercises.