FWIW, here's a completely unscientific, anecdotal piece of tid-bit with regard to preparing for long distance riding. I've heard this, and my experience seems to bear it out.

You retain endurance (slow twitch) conditioning much better than speed (fast twitch) conditioning. That doesn't mean that you can quit your 50 mile week-end ride today, and start again in February with no problems. It does mean that your legs will "come around" faster, and you'll be back up to your previous levels much sooner. The problem is all that Thanksgiving and Christmas weight.