I think it was Hal Higdon that has written online plans for various gaps between marathons, the minimum being 4 weeks. (I have a couple pinned on my wall at work so don't have the exact link.) e.g. the 6 week gap has a max of only 16 miles lsr and that on the sunday of the 4th week after the mara weekend, and that is when you are trying to get a bit of training in for the next one, so I wouldn't do 20 so soon.
You could adapt these plans or something else to make a maintenance plan. I believe it is better to keep a small number of mainly quality sessions per week rather than a few quantity sessions if you are going into maintenance mode. This is because VO2max is preserved better by the relatively high intensity sessions. Galloway has a book for all-year training but I haven't seen it.
Personally, once recovered I wouldn't want my maintenance plan to have gaps longer than two days and I wouldn't use long runs as a primary form of maintenance. A while ago I drew up a maintenance plan as follows: Mon 5 miles slow, Thur 4 miles tempo, Fri 5 miles slow, Sun 12 miles lsr. This gives 42 km per week. Maybe something like that would do, depending on your previous mileage and level of performance.
Posted: 21/03/2012 at 13:40