I think personally after training for a year and a half in muay thai and im on 5 years of boxing now i believe, i would walk away from a street fight even if i had my pride hurt over it. Its just never worth it. Most street fight situations when i was younger you usually end up stopping or winning becuase you or your opponent gas out in seconds. That would never happen to me now, and its very possible that you could seriously injure someone. But for some real advice...
Just kick the crap out of him. Literally, with kicks. Foot jabs, leg kicks, body kicks. You'll eventually frustrate him into becoming wreckless and then youll be able to punch him much easier. then continue with kicks!