I agree with what's written below in regards to marathoners. In terms of Pfitz and his books, they are probably more different at lower mileage levels, but become more similar to the way he training on the higher mileage plans, but I haven't spoken with him about it. RT mag had an article that said if you can't hit 70 MPW or more in training, your marathon will suffer, but many don't want training to take up that much time so they run on lower mileage but they enjoy the process more.
"If you've NEVER seen a marathon training plan without 2-3+ 20 milers, you're overlooking metric plans which top out at an equally round figure of 30 km, and also the Hanson's "marathoning for the masses" plan of a few years back: http://runningtimes.com/Article.aspx?ArticleID=4447
(b) Running doubles for high weekly mileage, the long run becomes relatively less important. Sure, if you're training for a marathon you probably don't want to skip it or anything; but if you're training well at 120+ MPW you could skip or minimize your long run and still run a good one.
(c) But the majority of marathon participants, training at say 25 to 50 MPW, absolutely need the long-ass long runs just to finish the damn race. 35 MPW doesn't prepare you to run a marathon; the weekly 15-20 miler is what gets low mileage marathoners to the finish line. And that 15-20 is a third to a half the volume for this population.
So not only do low mileage marathoners need to run a much higher % of their weekly volume in the long run, arguably they need longer long runs, period. At least they have a stronger need to run a few 18-20+ versus higher mileage more serious runners. If these long runs are as malmo likes to say the icing on the cake for the serious runner... well, the 35-40 MPW crowd never makes much of a cake in the first place, they're racing on pure icing, so they'd better not skimp on the icing. To refine that analogy, the low mileage marathon builds just enough of a cake to stack the icing on: the paltry mileage"