Do squats, press ups, sit-ups. Do light weights for ages and ages. That may help you to not lose too much of your fitness, while keeping your foot immobile.
I think that you acquire a base fitness anyway, after you've been running etc. for a while. I always find that no matter how long I take off from running within a week of getting back into it I can do 15 kms no problem - so don't stress too much about it!
