For me, that would depend on the terrain. If you have a moderately well developed base level of fitness, you can go a long way on the flat stuff just by pacing yourself, and taking breaks as necessary. You don't have as much control over your level of output on the hilly stuff. For you, probably the best thing would be to work back up to the 50-60 mile range, and then take a good day or two off before your event to recharge the batteries a little.