This plan doesn't list every run for the week; rather, just two key workouts. Some athletes will want to sprinkle in easy runs on the other five days; others will only do two other runs...it is up to you.
This plan trains you to become highly efficient at burning glycogen at marathon race pace. The goal of marathon training is to reduce the consumption of fuel at race pace. When you become more efficient at marathon pace, you burn less glycogen per mile and have enough to maintain your goal pace to the finish. First we need to build the engine with the shorter, faster workouts that many marathoners do only when sharpening for the race. Then, while keeping your mileage up, we will fine-tune the efficiency of the engine, with lots of running at marathon pace. The benefit of this approach is that you get your body as fit as you can first and then you get as efficient as you can. In the three weeks before the race, it's hard to get much fitter--you make only small improvements--but you can get your body used to working within a certain zone that improves how efficiently your body burns fuel in the race. This approach makes you very strong, so that a submaximal effort like marathon pace feels easy.
Your weekly mileage should be at least 20 to 30 before starting. If you feel overly fatigued or have acute soreness in a particular spot after the first week, this is a sign you're not ready. Back off and build your mileage a bit more. This program will be especially effective if you've recently been running short races.