http://www.halhigdon.com/marathon/Mar00novice.htmLong Runs: The key to the program is the long run on weekends, which builds from 6 miles in Week 1 to 20 miles in the climactic Week 15. (After that, you taper to get ready for the marathon.) You can skip an occasional workout, or juggle the schedule depending on other commitments, but do not cheat on the long runs. Notice that although the weekly long runs get progressively longer, every third week is a "stepback" week, where we reduce mileage to allow you to gather strength for the next push upward. Rest is an important component of any training program