The cornerstone of any marathon training plan is the weekly long run. Depending on how much and far you have run in the past, you’ll start with a long run--about six or eight miles--and increase it by one or two miles each week from there. Typically, every third week you should drop your mileage a little to give you some strength to surge farther and farther the next two weeks. Most beginners should top off at two or three long runs of 20 miles each before beginning to taper for race day.

When you do long runs, the general rule of thumb is to run them one to two minutes slower per mile than you plan to run on race day. The purpose of the long run is to get you used to being on your feet a long time, not to train for a certain pace. One of the biggest mistakes people make is to run their long runs too fast. Some training plans will tell you to pick up the pace a bit in the middle, or for the last few miles, to get used to running when you’re really tired. That may be fine, but if you’re a beginner it’s best to stick with just running those miles at a slow, even pace.

See also:

Speed Training
Training Plans