According to a study carried out at E. Caroling University as described by Owen Anderson in Peak Performance, experienced runners tapered for a 5K by running just 15% of their average weekly mileage in the week prior to their race. Each day during that final week they performed a descending number of 400 meter intervals at race pace: 8 x 400 the first day, 5 x 400 the 2nd day, 4 x 400 the 3rd day. On race day, the test group improved their 5K times by an average of 29 seconds! Who wouldn't be satisfied with that?
To taper for a marathon,Anderson, founder and editor of Running Research News, suggests starting out 4 weeks before your race with a 75%-50%-30%-15% plan. In other words, a month before your race, run 75% of your average weekly mileage, and the final week cut it to just 15%. He believes runners should drop any slow long runs as the race gets close, with that last week running a tempo-type descending interval program similar to what the E. Carolina runners followed.
Other research cited by Anderson has shown that as long as intensity remains constant, fitness can be preserved for up to 15 weeks despite a drop in volume by up to 67%!
I wouldn't be foolish enough to try holding 15 week taper, but before any marathon I would recommend following these principles recommended by Anderson as closely as possible. During the final 2 weeks, cut back on the easy long stuff and keep the intensity high with some short intervals.I would substitute 20-25 minute tempo runs if you are not accustomed to high intensity intervals.
Here is another twist. According to Anderson, researchers at the University of Western Australia have uncovered a new protocol for boosting muscle glycogen before a long race. Athletes following a typical high carbohydrate diet were asked to perform a 150 second high intensity session (130 % V02 max) followed by a 30 second all out sprint. The next day muscle glycogen levels were found to have increased by 82% over the day before!
So along with cutting back your distance to just 15% of your weekly average, Anderson suggests performing an all out 3 minute effort 2 days before your marathon, then filling up on carbs and taking the day before your race.
Anderson also believes it's a good idea to use the 3-minutes of hard running idea at the end of your regular moderate to easy workouts to prevent mid-week dips in your muscle glycogen.
Like anything else,this is something you should test before using prior to an important race.