As a runner, I often dissect and analyze what I've done. In training for a road marathon, I like to count the number of 20+ mile runs I've done leading up to race day. I find a simple correlation here, the more runs you do that are 20+, the better you will perform. Could my observation be an simpler? Sure, someone could debate this until they are breathless.
"You don't want to overtrain"-To this I say, start building up your endurance as soon as you know you want to run a marathon, look at marathon training as a larger project beyond where you currently stand and the next race. You shouldn't place a value on yourself as a runner or person in general based on how "fast" you run 26.2miles on limited training. If you run appropriately paced runs and build gradually, endurance comes over time. Patience is one of the most important ingredients to enjoying a successful training period.
That said, I'm eagerly looking forward to the rest of my training for Boston. I almost wish I had an extra 4 weeks so I could visit a few more long run venues that I so rarely get a chance at.