Duncan MacDougall, Ph.D. of McMaster University also studied tapering. Dr. MacDougall was working with 10-K runners, who would necessarily taper a shorter time, but the message for marathoners remains the same. Reduce quantity, but maintain quality. Continuing the thread begun two days ago quoting Dr. Costill's research, he found the main problem with tapering runners was that, psychologically, they worried about losing conditioning by running less, so became anxious. Also, at least in early time trials following a taper, they were so well rested and felt so good they ran the first few laps of a 5,000-meter test run too fast and faded in the final laps.
Rest is essential.
Believe me when I tell you, you won't lose conditioning when you taper. On race day, however, you're going to need to control any impulses to run the first few miles too fast. Maintain the same pace as your usual weekend runs. One of the reasons for these long runs is to get your body used to moving for a long period of time, as well as distance. If you cut the time length of your workouts short by running too fast, you spoil the effect of the taper and don't get full benefit.