Practical Tapering

1 Short Race Tapers

For a short race (5K) it seems that drastically reducing training volume, while keeping intensity high can produce great gains. Based on [1], a 7 day taper that reduces mileage by 85% and has a decreasing number of hard intervals (7 the first day, 6 the next, etc) produces good results. However, this type of taper tends to cause muscle soreness, which makes it less than ideal for longer races.

2 General Running Taper

The consensus of 27 studies for tapering is listed below [2]. These guidelines are not specific to a given distance and I found no studies looking at marathon distance or greater.

Do not decrease training intensity (intensity seems to be key)
Reduce mileage by 20-60%
Taper for 8-14 days. Some athletes will do well on other durations, but some will do worse than no taper.
Reduce mileage exponentially
Keep training frequency the same may be better than reducing frequency.

3 Fellrnr's Personal Approach to Marathon Taper

The following is my personal advice based on anecdotal and personal experience. This approach works well for a marathon where you are focusing significant resources into an optimal performance. Obviously, running the marathon distance (or greater) does not require a taper, but performance is optimized by doing one.

Taper for two weeks
Cut out any easy paced/recovery/junk runs (if you are doing any)
Have the last long run at the beginning of the taper. No run past this point should have the purpose of improving endurance.
Avoid hard downhill running in the taper
Do medium length runs at marathon pace (Running at marathon pace improves your sense of pace and become comfortable at this speed)
Do 'easy intervals' - for instance, mile repeats at tempo pace with full recovery. The idea is to be fast enough to keep prevent detraining, but easy enough to avoid any muscle soreness. You could do harder intervals if you are confident they will not cause soreness.

4 Fellrnr's Personal Ultra Tapering

Like many ultrarunners, I do far more races in a year than typical marathon runners. This race load means that a longer taper is impractical. Therefore, for a short ultra (up to 50 miles), I take the day before the race off completely and convert that week's hill training to a flat run. For longer races, I'll reduce my Monday and Wednesday runs to one hour rather than three, avoid hills and take Friday off. (I normally only run Monday, Wednesday, Friday and Saturday.)

5 References

[1]The effects of taper on performance in distance runners http://cat.inist.fr/?aModele=afficheN&cpsidt=3379866

[2]Effects of Tapering on Performance: A Meta-Analysis http://journals.lww.com/acsm-msse/pages/articleviewer.aspx?year=2007&issue=08000&article=00019&type=abstract