Add intensity if you don't have time for length! I had a blast on the Cap to Cap Century (flat) with so little time to "train" it was borderline crazy. In the 2 months before I did follow Chris Carmichael's idea (not his actual training plan) from "Time Crunched Cyclist" to do intense intervals, hill repeats etc to make the most of the few hours I had to ride. I was pleasantly surprised with my time and how fun/easy it was...in retrospect I probably over did the hill repeats but I am now having some super fun pay off mountain biking ease in Charlottesville (: