I did 203 miles at the 12hr Calvin's Challenge with only two training rides longer than 45 miles during the 9 months prior to the race. My secret - short interval rides of an hour or less twice a week. Very hard interval work gives you the fitness to make it easy to cruise at 20 mph. Doing hours and hours of moderate pace endurance work won't be able to give you that with any degree of time effectiveness, if at all. So yeah, add some intensity (and subtract some milage - the trade off is worth it.).