I just watched Caleb try to hold Mack down on the couch. Couldnt do it. Even with all of Calebs 275lbs on top of him and pressing down on him, and Mack being nice and not biting or anything, couldnt hold him down.
This confirms my opinion that I DO NOT want to tangle with any large dogs if I am empty handed, particularly a 100lb pit bull. I think you are right, find a tree, and climb it fast.