Literally thousands of military personnel have been successfully trained and have performed with significant exercise before breakfast. In fact, I would say, based on real life experience, that those who have no other medical issues and must eat before exercise are the abnormal ones.
Not only is significant work before breaking a fast normal, it is a valuable attribute. If you have trained your body to eat before stress how in the world are you going to handle the unexpected if it happens just before your scheduled meal? Accidents, earthquakes, fires, falls, etc. don't happen around your meal schedule.