I work out a lot, when I can. I see it as if I make myself stronger the weight will be less of an issue and I'd be less likely to injure myself on a shoot. But being careful, as to not injure myself in the gym.
I have found lots of core body exercises work a treat and really help prevent doing my back in (swiss ball). And timing of working out so I dont ruin my body before my next job - which is difficult to do though when things come in last minute. If I know I'm likely to not work for a while (i.e. Christmas) I'll really chuck more weight around and do some higher risk exercises to build up overall strength (dead lifts, squats, clean and jerk etc...)
I like the gym and also like the physical element of working in sound/ booming - beats being stuck at a desk which I believe is probably worse for your physical wellbeing and health than using your body in your work. But I'm 25, so ask me again when I'm 50!