I think your body will get used to whether you eat or don't eat before working out. I don't eat before working out because I take thyroid medicine when I first wake up in the morning and I am supposed to wait for 60 minutes before eating anything. So I wake up, take my meds, and head to the gym. In the car, I drink a diet pepsi.

I usually work out for 75-90 minutes. My workouts usually include a 3-6 mile run on the treadmill, plus weight lifting and body weight exercises (lots of push ups, pull ups, squats and lunges). On Saturdays I do a long run of 10-12 miles and don't eat before that either. I've heard that working out on an empty stomach teaches your body to burn fat. Not sure if that's true. I've also heard that working out on an empty stomach means you aren't properly fueled and can't work out as hard as you should. I can say that without a doubt that is not true, at least in my case. My body has adapted and I feel great and full of energy when working out. It definitely wants to be fed when it gets home, though.
Pam