How to do TaperingThe longest mileage day in training for a marathon or half marathon should be two to three weeks before the event -- preferrably three weeks for the marathon. That day should be 20 miles or a little more for a marathon and a full 13 miles for a half marathon.
Then the racer reduces their mileage to half of the race distance for their long mileage day in the two weeks before the marathon or half marathon. For a marathon, that should be no more than 10-12 miles. For the half marathon, no more than six miles.
Why Taper for the Marathon or Half?Giving the body two to three weeks to heal and repair after the longest mileage day ensures the racer is in race-ready condition on race day. This gives the body's energy systems a chance to fully restore. It allows the muscles to repair and rebuild. It also gives time to fully heal up any foot blisters.
Sleep and CrosstrainingRacers should try to get full nights of sleep during the tapering period. While reduced mileage may make them antsy to do more speed or to work on strength training, it is wise to stay with moderate intensity exercise only and not work on building more muscle. The taper is needed to give your body a chance to rest and repair so it will be in top form on race day.
Reference: Bosquet, Laurent; Monpetit, Jonathan; Arvisais, Denis; Mujika, Inigo: "Effects of Tapering on Performance: A Meta-Analysis. Medicine & Science in Sports & Exercise. 39(8):1358-1365, August 2007.