Actually, a fit man would murk a pit. It would probably have to get to your throat to kill you. Your hands and arms would be fukked up. No way around that.
IMO, it would take a mastiff, Rottweiler or large German Shepard to kill me, and I still wouldn't bet on that.
I'm 6'2 230. If you're light in the ass, you could probably get done in by a mean Lab.
What you're ignoring is the fear factor.
An enraged canine is not afraid of you, at all. Meanwhile, even if you're in shape and even if you're prepared, you're still going to be terrified of
.
So the fight starts. And as you've said, he's gonna fukk up your arms, hands, probably your legs and chest and whatnot and he's not gonna stop until he's called off or you're dead.
So you're bleeding from a couple dozen places, probably missing some appendages, pissing yourself, smelling like fear and p*ssy and you're gonna what... punch the dog?

You do know that a number of canine breeds aren't dissuaded by pain, at all, right? And they have jaws strong enough to generate about 330 psi of pressure. Your bones are crushed, you wouldn't even be able to form a fist, breh.