You can do anything fixed.. just depends on your gear ratio, how fast you want to go and how much you want to mash up hills and/or spin back down.

You best bet is to do some medium-length rides fixed and see how you feel. The bike rally has rides every weekend starting on the 14th. The first rides are around 35km and build up to ~130km. Or obviously you can just do the rides solo as they post all the maps/cue-sheets (including Google Earth .kmz files: http://www.bikerally.org/maps.html).

There's some rolling hills North of the city that get tiring on my road bike, so it's nice to be able to coast down the other side (or gear down and crank it to get lots of momentum for the next ascent).