The hills on the PBP aren't long (most of them), but they are steep, and they go on for the entire 1200K. The whole ride is uphill, downhill, uphill, downhill.
I was extremely happy with my low gear of 30x34 (chainring: 53-42-30; cassette: 11-34) ... by the end, I just stayed in the 30 and flipped a bit back and forth in the cassette.
I will mention though that I did very little hill training before the PBP because I lived in Manitoba where there are no hills. I'm still working on learning to climb the things.
But there are people who do the ride on single speeds and fixed gears!!