Congratulations on completing a century.
Having done many myself (I am 64), I have found them to be a good training for longer distances.
Your next task is to complete a Super Series of Randonneuring Brevets (i.e., 200 km, 300, 400, and 600 km).
Perhaps you will be among the 5000 riders of the Paris-Brest-Paris (1200 km) in 2007.
That is the ultimate ride of long distance riders.
Cheers ... you've made a good start.