3400 sounds reasonable for a regular bike. Burning 750 calories per hour is fairly typical. 850 is a bit high, but not out of the ordinary.
As far as the body surviving a 3400 calorie burn, it's no big deal. One pound of fat contains 3600 calories. Your body stores about 2000 calories of glycogen in the muscles. Food eaten supplies additional glucose that can be used by the muscles. You can burn much, much more than 3400 calories in one event.