Gordon T -- I'm finding myself nodding in agreement with your long run comments, based anecdotally on my own experiences. For me, long runs (18-20 miles) really seem to boost my fitness, no matter what distance race I'm training for.
In my half-marathon build last spring I worked up to a 20-mile long run. Perhaps because I am low-mileage overall, the added miles simply are the key. But I think those single 2- to 3-hour runs uniquely contribute. Of course, I ignore the "25% rule," as a 20-miler is typically closer to 50 percent of my weekly total.
For marathon training, I like to include one 23-mile depletion run, usually my next-to-last long run (other six long runs being 18-20 miles). No calories before or during. Probably no good reason to do this, just an old ritual I maintain (old theory was to practice switching to primarily fat-burning).
When I get back to my office, maybe I will post last year's half marathon training, showing how I scheduled the long runs. Interestingly, I also set a lifetime 5K age-graded PR during that training -- a few days after the first 20-miler in the cycle.