Not run. Turn around, bark back and stand ground aggressively. Unless it's a dog that can kill you. Then do same and shoot it if you have to. Anything else just kick it's a$$ till it gets the point.
I doubt she was big enough to kill me but she definitely could have taken a few chunks out of me. Especially while I was wearing gym shorts. I didn't want to find out where she was going to bite first but she definitely wasn't playing around.