When I had an extended period off a couple of years ago with an ankle injury sustained on the Carneddau I found, while I couldn't run beacuse I couldn't weight bear, that I could do freestanding deep squats coming up onto my toes each time. Gradually built up till I was doing hundreds (4 - 500 at a time at a regular beat - try it). I was surprised how little I'd lost when I got back out on the fells. I actually think they are a great exercise but hardly ever do them when I'm running because you can't do everything. Apart from these I agree with others about core exercises.