What does it mean to taper off for a marathon? Well, the main idea is that you gradually reduce your training volume over a certain period and at the same time adjust you diet for maximum energy storage. By tapering you squeeze all energy to be released on race day.
How to Taper
Most professional runners and coaches agree that the real tapering period for marathon runners which includes changes in the main diet should start three weeks from the race. About three weeks from the race you will have had your last long run over 18 miles. Long runs decrease to between 12 and 16 miles at two weeks and between 8 and 12 miles at one week from the marathon race day. During these three tapering weeks you should avoid racing and speed training. The running you do are easy runs and conditioning runs, because during these three weeks there is not much you could do to improve you finishing time anyway. Tapering off also means you should reduce your weekly training volume steadily to allow you muscles to heal and store the most possible amount of energy.
Also you will have to change your diet in order to store as much glycogen in your muscles as possible. By increasing the amount of carbohydrates you consume every day you are sure that all glycogen stockpiles are filled to the top. Having consumed high amounts of carohydrates during the tapering off period you can postpone hitting "the wall" at the final miles of the marathon race or avoid hitting the wall completely. During regular training a runner consumes about 50 to 60 percent of their total daily calories from carbohydrates. During the three weeks of tapering off you should consume about 60 to 70 percent from carbohydrates and the last three days 80 percent of your calories from carbohydrates.
While tapering most runners experience running effortless and would like to go faster, however, restrain is needed. Running too fast during tapering you would risk throwing away much of your training efforts from provious months. Going too slow is better than going too fast.
During the last week of tapering don't eat unknown food to avoid upsetting your stomach. Also avoiding viberous food during the three days before the race is a good idea to keep your stool stable which is very preferable during your race.