I've done several Tour de Cure (diabetes) and MS rides. They are non-competitive with plenty of rest stops, so I don't do a lot of training specific to those rides. In the two months leading up to the rides, I try to do one progressively longer ride a week with at least the last couple being centuries. On other riding days I do some hill climbs and sprints mixed into mid-distance rides two or three times a week. Just as important is to take a couple of easy days (nothing but relaxed recreational riding, or off the bike entirely) at least once or twice a week to allow adequate recovery. Overtraining is just as bad as undertraining, so make sure you plan for adequate recovery. In the last days before the event, take it easy. If it is a choice between one more day of intense training or one more day of rest, choose the rest. Hard training stresses your body to trigger adaptation, the actual improvements come during the recovery phase.

Good nutrition and plenty of sleep are very important. Also pay attention to your on bike hydration and nourishment, don't try something radically different on the day of the event, everything should be very familiar and well tested.