Here's an eight-week training guide for Cycle NC, a week long bike ride across North Carolina that I'm considering doing this year (although the route is not the greatest). I've done weeklong tours and rides, and I just try to get out and ride a few times during the week with a longer ride or two on the weekends. I'm sure that if I were a bit more systematic, however, I would be able to go faster and farther.

http://www.ncsports.org/cncdetail1.php?id=23