A taper is a reduction in total miles run in preparation for a major target race.  It is a reduction in duration, but not necessarily intensity, of those runs.

Before marathons, a taper of 3 to 4 weeks is generally recommended.  That is, the trainee will run at the same pace, and under the same conditions, as the rest of his or her long runs during the taper period.   But the length of the long runs will gradually diminish, to the point where the week before the target race, the trainee only runs a few miles for a “long run.”   And the duration of midweek runs will also decrease.

Most marathon training programs (including MCP) build in a 3 or 4 week taper back from the race day, in determining when the last long run is scheduled.   [Building a marathon training program is largely a matter of working backward from race day, and allowing for a progression of long runs followed by a taper of sufficient duration.]

After a full season of training, doing a lot of running right before the target race can’t really improve performance.  But it can definitely hurt it.  

After every long run, your body needs time to heal, to recoup, to repair all the micro-tears in your muscles.   After your 20 mile run, you will need more down-time than ever before to get back to the point where you can run *longer* than 20 miles.   On top of that, your body has been pounded by successively longer training runs over a full four, five or six months of training.   The taper period allows repair of that cumulative damage, allows for rest prior to the marathon effort. 

During the taper, your long runs, or your midweek runs, may seem easier.    That’s good.   Resist the urge to run longer or faster, and reign in your energy.   The desired goal is to be healthy, rested, and well fed and hydrated going in to race day.   If you’re just itching to do a long run by Oct. 26, then you have wound up in the right frame of mind for tackling the marathon.