Most plans will tell you to do your last long run 2 weeks before the race. Then gradually cut your mileage up until race day. It feels weird, and may test your confidence to drop the mileage, but it does work. Last time I did a 12 mile two week before, then my normal mid-week runs, then an 8-miler 1 week before and my normal midweek runs and two full days off before the race.
Experts have debated over the exact number of days needed for a successful taper, but it is certain that the focus shifts from quantity to quality during this three to four-week process. Incorporating speed work, like the
followed by a light day will help maintain confidence and prevent a common side effect, discussed by
Active Expert, Gale Bernhardt
in her article,
"The Taper Blues."
(http://www.active.com/story.cfm?story_id=13135&category=Triathlon&num=0) The blues can easily discourage an athlete who has become accustomed to high-energy expenditure during training because he or she will have more energy and feel stronger than ever before. It is imperative not to act on these feelings which may sacrifice months of hard work. Also, it is quite common for an athlete to overlook the fact that they are burning fewer calories; therefore they most adjust their nutrition plan accordingly.
Your last long run should be three weeks before your race. Two weeks is too close. I have been fortunate enough to train with many great runners, some who have qualified for the Olympic trials in the marathon. They all agree with three weeks. Start tapering after your last long run by running approx. 75% of you max milage the next week, 65% the next week , then approx 40-50% the last week. That will give you the best chance to be fully rested from the long runs and fresh for your race.