uoft23 saidI bike 40km six times a week, 3 of which I do sprints and go as hard as I can and I've been making steady progress with my speeds as well as at the gym.
Just eat eat eat and then eat some more
I recently completed the Friends for Life Bike Rally from Toronto to Montreal http://www.bikerally.org/
(about 112km per day), during the beginning of training rides in April, I was clocking in at 22k-25km p/h by the time the ride left in end of July I was closer to 35km p/h and the last 3 days of the ride was clocking an average of 40km.
Like uoft23 I do about 40-50km two times a week, and combine sprints and distance. Just vary up your training rides: one day is pure sprints and the other is pure endurance (trust me you will need that aspect).
I also did the RJ Muscle-Building 12 week program http://www.realjock.com/workout/901/
and did two cycles of it (6 months) prior to the actual ride and that program combined with training rides got my body in the shape it needed to do 100km rides back to back on weekends for training as well for the actual ride itself. And eat, eat, eat and then eat some more as was stated but try to be 'clean' about it. And drink water water only. No juices and skip the gatoraide except on your rides (and water that down 1/2 and half).