"... the marathon without hitting "the wall"-- I can tell you what it will take to get over that hump as I've been in that exact same boat as you but was able to figure out how to get out: (larger volume/base-- avg. 50-60 mpw for a few months, then Pfitz 70 mpw for 12 week program, add several miles of goal MP running into the mid-week med. long runs, run long runs on "empty", without carbo loading or caloric intake, just water, run easy on recovery days, be honest and do tempo effort the full 7 miles of the prescribed 12 mile workout that day, stay healthy, do approx. 1/2 of long runs finishing the last 3 miles at MP, 1/2 MP, 10k pace. Most long runs can be 17-20. Do one long run of about 22-23 one month prior to goal marathon). Carbo load appropriately (not too much to gain weight), and fuel pre race morning and during race appropriately."
Above is some from advice from an internet board. I've come to most of these conclusions on my own, but have yet to reach others. The 12 w/7 @ tempo just seems impossible for someone of my limited speed and I've never figured out how to "carbo load appropriately" or "fuel pre race morning."