If you have time to change your gearing a bit, maybe think about that (if you're geared high right now). I've found that 70-74 gear inches is good. Spin for all you're worth on downhills, sprint the short uphills and good luck on the long climbs. Your ass will be in serious pain after that distance, so maybe it's time to be absolutely certain your position on the bike is ideal and invest in some chamois cream (aka: ass butter). Ya, ass butter. I know, but it works (cue Last Tango in Paris image).