I just finished a 240 mile fleche with three other riders, all of whom have ridden fixed for distances of 125 miles or more. Only one of the riders did last weekend's ride on a fixed gear though. I believe his bike was setup for 72 gear inches, but he's also fairly young and his knees are still uneroded.

In general, the one terrain feature that endurance fixie riders seem to dread are long descents. Nobody *****es about having to turn pedals on flat terrain, and even steep climbs don't draw too many complaints, but it's always the long fast descents that inspires folks to pine for a freewheel.