Think about the fact that most all of the best marathoners in the world have come from a background where they worked their way up to the marathon distance. Most conquered the 5k or 10k then slowly moved to the marathon later in their career. Not too many of them do it the other way around.
Randy, or anyone else, if you are interested in an unpublished article regarding getting faster, I would be happy to email it to you.
danm at wittkieffer.com
The article examines two individuals and how they can get better based on their body makeup. Specifically how one runner who was a typically slowtwitch kind of guy could get faster at the half marathon distance.