For some reason, I've come to believe that the multiple mid-week 13-15 mile run is the staple of marathon training.
Sure, the long run is important. You have to learn what happens to your body at 20+ miles. You have to learn to hydrate and fuel during your run. You have to build endurance. Don't skip the long run. And you better practice marathon pace at the end of some of those long runs, too.
Likewise, the tempo run is a must. You'll never learn to hold a difficult pace for mile after mile if you don't practice it. And the physiologists (who are much smarter than I) claim that lactate threshold is the single best predictor of marathon performance. Tempo runs aim to improve lactate threshold. You'll never be able to run your fastest marathon without a steady diet of tempo runs.
But the mid-week 13-15 miler seems to be what gives my marathon training its kick. Perhaps I say that because I enjoy them (sick, I know) more than the other types of runs. But I know that twice a week I'm going to pound the pavement from 5:00AM to 7:00AM. I'll suffer some misery, but then I'll see the sun rise and for some reason I'm not as tired. These runs seem to separate those who train for marathons from those who train, but happen to run marathons. If I want, I can mix in marathon pace miles. If I don't feel like it, I just run at a long run pace. And people look at me funny when I'm wearing my reflective vest in broad daylight. Yeah, I've been out here that long.
Today I belted out a 15+ miler at 7:35 average pace (minus the warmup 8:50 mile) and my heart rate never climbed out of the 150s. Gotta love cooler weather. Aerobically, I feel like I'm in pretty good shape. I could certainly improve, but my HR seems to stay low for a given pace. Old man age seems to be catching up with my legs. I can't get them to go much faster. It's really weird to feel like I'm at about max effort in my legs while I'm able to breathe and converse normally.