Yes, a 200k at 49x17. The distance wasn't so much of a problem as the hilly terrain (and the rain and 25+mph headwind). If you can scope the course out ahead of time, at least the general topography, and select your gear ratio appropriately (maybe even use a fix/fix hub) undulating terrain should not be too much of a menace. But reconnaissance takes a little excitement out of the experience. I also did this with a messenger bag, which I plan on leaving at home next time. Personally, for 400k and over I'd be strongly inclined to using gears, as the sense of accomplishment of doing distance fixed seems to diminish in direct proportion to the aggravation of doing distance fixed.