Please note there are many different training programs. This is a broad template and should be adjusted to fit your ability, health, body, training program and goals.
The first 8 -10 weeks, called Base Training, should prepare you and build up to your long run. For example, week 1 should be two miles; week two should be three miles and so on until the next phase of training.
Around week 10, runners begin Endurance Training and can usually manage 10-12 miles building on that every other week. Using this model, week 10 is 10-12 miles, week 12 should be 13-14 miles, and week 14 should be 15-16 miles, working up to 20 miles. The odd weeks should be between 7-10 miles.
Typically, runners start tapering approximately three weeks before the marathon, though many people find this difficult, becoming antsy for the marathon. Remember, tapering allows for the body to rest. You should continue your cross training and maintenance runs during this time.