Part of running a successful marathon is picking the right race. Your first priority is to allow yourself enough time to prepare. I know of no formula, but my best guess would be that you need a minimum of 6 months to prepare if you are currently running 20 miles per week, 3 months if you are running 40.
One of the best sources for marathon information is marathonguide.com.
Find a race profile that suits you and your budget. Look at weather (potential for heat) and course profile (hills or flat).
Register early, get your gear lined up; shoes, etc. and get started!
"There is no better motivation to run than a marathon looming in the not too distant future."
(c) Dave Elger 2008 All rights reserved