My last long distance fixed gear ride was the LA Grand Tour Highland double century in June. This was a hilly ride (10,000 ft) and the descents kicked my butt. The constant spinning and braking, even with both brakes, on the long descents absolutely drove me nuts. Next year, I'm doing this ride on a single speed, start to finish....why torture myself?....
Doing centuries with roadies shouldn't pose much of a difference from riding your geared bike as long as the route is relatively flat.