I haven't done a ride that long yet, but if I may comment as an exercise physiologist/biomechanist, what you don't want to do is a really hard (for you- whether that mean fast/long) bout for about 3-4 days before. Every time you exercise at moderate to hard intensities, you tear muscle fibers. Don't freak, it's natural. Most muscle will repair itself within 3 days, so I'd want to stack the deck to having my muscles fully healed before a high intensity bout. Stretching and low level workouts are good to keep the muscles more adaptable, but you don't want to go into the race with damaged muscles. Some of this damage you won't feel, but I'm sure we all have felt what happens when you decide to overexert yourself with no training specific to whatever it was- the dreaded DOMS (delayed onset muscle soreness, aka weekend warrior syndrome). The most likely culprit for causing DOMS is micro-tears in the muscle and the immune response to the tears. But there also maybe be an electrolyte component- so stay hydrated! Good luck!