Absolutely avoid fighting if you can. No matter how good you are there's still a chance you can get seriously hurt. Walk away, or run if you have to. Pick up poop from the ground and make them think you're crazy or just leave you alone (story from my Judo sensei).
Punching is like throwing a baseball or hitting a tennis ball. If you watch carefully, they put their entire body into it. Everything is a smooth motion. They're not just using their one arm. Even for (good) punches where it seems like they're not moving much, they're building a lot of power from the rest of their body.
If you are in a situation you cannot avoid, stay calm and see how to get out of it as quickly as possible. Play dirty if you have to, but get away. If they're going to fight you, they're not going to play fair anyways.
In general, I recommend to be in good overall physical shape. Being fit will allow you to run faster and potentially if they see you're in good shape they'll avoid picking on you and fighting you. Take some self-defense classes. I found Judo to be a great exercise and sport.
Outside of good luck finding a pile of crap in most places someone starts with you, good advice.
Very few of the fights I have been in I could say I wasn't hurt a bit and very lucky not to have had a major injury. A lot of the most buff fighter types end up losing to the end of a bottle, knife, chair, or worse. Most antagonists have nothing to lose that are serious. Those that are just mouthy bitches usually go away with intimidation or easily with a choke or slap.
Growing up I ended up in clubs early on, like 15-16...I looked older and met older chicks. Along with this came the cro-magnon types that didn't like a teenager stole their dance partner.
Shit got serious for a couple years when I pissed off some local gangs. It culminated into me going away for college and losing everyone's number that could be a link back. I had been shot at more than once. Beat with boards and other weapons...fortunately I knew how to defend myself and always got away mostly better than worse.
If you are truly confident and have no problem unleashing that haymaker should you need too, usually most fights end fast. If you allow a guy to push you around, it tends to perk up those around you. As soon as things go south expect sucker punches and random kicks.
If I know shit is coming down, I go and find another place unless it's petty. No matter how good you think you are 2-3 on one is a losing battle in a bar/club usually and quickly turns into 10 on one with opportunists.
One of the biggest cards you can have in your sleeve, is to simply get to a bar a little early and chat up the staff. Show the bouncers and bar staff you are the even-headed, normal dude that just wants to get laid/have fun that night. Usually they have your back.
Also some dojos have 'special interest' classes you have to ask about. My instructor was world ranked (and later ended up transfered due to demand) and did workshops for rape victims that worried about repeat attacks, guys going to prison/getting their asses kicked daily, etc.
I will say real defense type classes aren't for everyone. There is usually pretty good contact.
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