Steve,
You will probably get a lot of response from some folks a lot more qualified than I. Here are two _link_ that may be helpful...
http://bicycling.about.com/od/trainingandfitness/
a/century.htmhttp://www.ultracycling.com/training/century.htmlThe
Second _link_ is the most helpful. I used one of their plans more or less to do a 3 day 250 mile ride - it all worked out good.
You mentioned not going over 50 miles - I looked at my log - did not much over 60 miles for training but did do a lot of winter time spinning.
Joel