How to handle physical pain in a fight?

i really need to know how to tolerate physical pain in a fist fight, particularly being punched in the face. no, i don't like fighting, i try to avoid it at all costs, but when it's unavoidable and there's nothing i can do to stop from being involved in a fight, i fear that i'd probably end up losing, due to never been in a real fight, and i'm not sure how much pain i can tolerate.