What a Taper Is Actually Doing
A taper isn't rest -- it's a deliberate reduction in training volume while intensity is mostly preserved, designed to let the body clear accumulated fatigue from months of build-up training without losing the fitness adaptations that build-up produced. Fitness and fatigue are both real and both temporary; a taper is timed to let fatigue fade faster than fitness does.
This is why a well-run taper leaves most runners feeling both undertrained and strangely fresh in the final week -- both sensations are accurate at once, and neither one means the taper is wrong.
How Long, and How Much to Cut
A 2023 systematic review and meta-analysis in PLOS ONE concluded that a "≤21-day taper, in which training volume is progressively reduced by 41-60% without changing training intensity or frequency, is an effective tapering strategy"[1]. Mayo Clinic Health System's guidance draws on the same underlying research (a 27-study meta-analysis by Bosquet and colleagues) and recommends marathoners taper for roughly 19-22 days -- about two to three weeks[2].
In practical terms for a typical marathon block: if peak weekly volume was around 80km, a taper following this research would bring that down to roughly 32-47km by race week, cut gradually across the final two to three weeks rather than dropped all at once.
What to Keep: Intensity Over Volume
Both sources are specific that intensity and frequency should stay largely intact even as volume drops -- this is the detail most runners get backwards, cutting hard efforts first because they feel like the riskiest sessions to keep. Short, race-pace or slightly-faster efforts within an otherwise easy week are what keep neuromuscular sharpness and running economy from fading during the taper.
A practical taper structure: keep the same number of weekly runs and roughly the same intensity distribution, but shorten each run's distance -- a hard interval session becomes fewer reps at the same pace, not the same reps at an easier pace.
Common Taper Mistakes
The most common mistake is treating the taper as full rest rather than reduced training -- stopping running almost entirely in the final two weeks removes the intensity stimulus the research says should stay, and can leave legs feeling flat and unresponsive on race morning rather than sharp.
The second most common mistake runs the other way: getting nervous about lost fitness and squeezing in one more long run or hard workout inside the taper window. Race-week panic training doesn't add meaningful fitness this close to race day -- the adaptations from a session run 10 days before a marathon haven't had time to be absorbed -- but it does add fatigue the taper was specifically designed to clear.