Okay, so I have never been in a fight, so I have no idea what it feels like to get punched, and I’m afraid that this will limit my writing. Any helpful tips?

Well, the short answer is, it sucks.

Getting hit is one of those things you sort of need to experience for yourself. The best advice I can give you on that subject is finding a friend you trust, and who knows what they’re doing; to sucker punch you.

Alternately, start taking classes in a martial art that appeals to you, sooner or later, you will get clocked by accident.