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. 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.
Marathon running is a psychological, as well as physical, game.
That's because in building to a mileage peak, they often overdo it and temporarily suppress their immune systems. Marathoners thus are more vulnerable to any viruses they might encounter. To avoid colds, try to avoid people who have them. And get plenty of rest. Hal Higdon's Smart Running is a collection of questions and answers from his on-line Ask The Expert column.