Nash,
Let it get your arm, then get it on the ground with your weight on it and use your other arm on its throat. If you can't do that improvise. I was only half joking about biting it's throat. Go for its eyes, nose whatever, hell shove your fist down its throat, but fight don't run. Running will almost always make it worse.
Even a big stick is better then your hands.
I would never go fishing out in the middle of know wear without some kind of weapon, preferably a gun. Unless of course its a state park or local park then its ilegal. If you get out of the city it is just prudent, rattle snakes, mountain lions, bears etc. Hell even a park in the hayward hills has mountain lions.
Nature may need saving from man, but thats large scale. On the small scale many of natures criters are a much bigger threat to you then the other way around if you run into them on a bad day.
Fighting a pit would suck, they are all muscle.