Two Steps Forward One Step Back: Marching To Marathon Success
The 10% rule of limiting weekly increases in mileage to no more than ten percent is widely quoted as the sin qua non of incremental training. In many cases even this may be too much. An invariably rising load of 10 percent 
each week may not give your body time to recover from the stresses of running.
In marathon training, I believe that for beginners, the long run is the key to successful completion of the marathon. Many years ago, in a very early survey, it was found that if an individual had completed 3 long runs of 18 miles or more (for example two 18 mile runs and one 20 mile run) the chances of completing the marathon were well over 90 percent.
Good questions are: 1) how to get to those long distances and 2) how long should your longest run be. The reason we ask these questions are to both be successful in completing the marathon and to limit injuries and enhance recovery.
I like what I call a “two steps forward, one step back” approach. I recommend this as a guide to the long run, up to about 17 or 18 miles. What this means is that you move up gently in your long distance running mileage for two weeks, than have a considerably easier and shorter long run. Once you have reached a long run of about 18 miles, you take two easier weeks to allow your body a good recovery. And the long run only has to top out at about 20 miles for a successful completion of the marathon. Two to three other days of running and one to two days of cross training would work well with this approach to the long run. There are some programs that advocate an approach like this, while a good many just keep moving up the mileage. For beginners, I favor this gentle approach with recovery that is built into the schedule.
Weeks until Marathon : Long Run Distance
19: 8
18: 9
17: 10
16: 6
15: 12
14: 13
13: 8
12: 15
11: 16
10: 10
9: 18
8: 10
7: 12
6: 18
5: 10
4: 12
3: 20
2: 12
1: 8
Marathon: 26
