This this this this this.
Moraghan speaks the truth (as usual). Gaining marathon potential over short distances allows you to fulfill your ultimate marathon potential later in life.
If I can use myself as a rather self-indulgent example 
Over the last 2 years since I started running I've done 3 HM's and was going to race the Kilomaton. It's easy to see where the next step was. However for the next few years I'm focusing on the 3k & 5k. I have no plans on racing a Marathon now until I'm at least 30 (which will be 10 years from now).
Essentially my aim is to maximise my race times over the shorter distances (or, at least get as close to them as possible) before moving upwards. 5k leads to 10k leads to HM leads to Marathon.
I'm currently base building but when I start my first 5k mesocycle I'll be doing upwards of 70mpw. My initial planning's putting me on about 8 - 10 miles of quality per week during this time, which means a full 60mpw will be done at an easy aerobic pace - which, in addition, will hold me in good stead for marathon efforts years from now - you can argue I'm base building NOW for marathons.
The one aspect in which I deviate from Moraghan's principles are the two long runs, which max out at 15M/12M for each for the 5k. However as I increase race distance focus I would expect the midweek run to remain constant and the long run to increase upwards in parallel with increased mileage and race distance focus.
As a final note, as you increase race distance you will be developing more and more the systems on which your best marathon times will be run - as you progress from 5k to HM more aspects of marathon running enter your training. By the time you reach marathon training you should be ready to do yourself extreme justice.
Thanks for taking the time to write this.
I'm not trying to be all "high horse" about this, it's just something I feel strongly about is all.
Edited: 08/10/2010 at 12:49