According to the Hanson Brothers, it takes approximately 10 days to see adaptive results from a hard training session. So any challenging training within 10 days of a marathon will not produce any new benefit by race day.
Because non-injury muscle damage from harder workouts can take up to 14 days to heal, working out hard or long enough to cause that damage within 14 days of a marathon only hurts your performance since the damage may not heal in time.
Therefore you want to avoid any challenging workouts within 1.5 weeks of a marathon, and want to avoid any lengthy, damaging workouts within 2 weeks of said marathon. Basically, no long runs or max intensity speedwork within 14 days, and no speedwork within 10 days.
The big mistake most make with a taper is to take more days off than before and to reduce intensity along with reducing volume. They go too far with tapering by tapering everything about their training, instead of just the volume.
By various accounts, most who actively taper get to the line practically rusty from sudden, extreme undertraining or lack of training. You still want to train as regularly as before, just without all of the volume or any high intensity.
Of course, you don’t want the shorter runs to be TOO intense, or else you’ll do damage that won’t be repaired by race day. So the key is to do less volume, but at a greater (albeit minimally damaging) intensity than before.
Ideally, your easy running has been at a truly easy pace. If you do your regular runs at too hard of an intensity, it’s harder to to taper at a higher intensity without entering a danger zone. But then again, if your regular runs were indeed too intense, you probably have gotten injured or burned out, or been forced to take many days off, between week 1 and the taper.
The ideal intensity middle ground? Marathon pace! The Daniels’ theory is that during a single workout you can handle M-pace at up to 20% of your weekly training load. It should be a challenging but easily sustainable pace, what some would call medium or moderate intensity, and during a shorter taper run there’s no way you should get anywhere near the Daniels’ recommended 20% volume max.
If you’ve done marathon tempo runs during your training, none of your shorter M-pace taper runs should be any longer than the shortest sustained tempo run. Otherwise, a good imposed limit for any M-pace taper runs is 30 minutes.
If I have a planned mileage for a day that would take longer than 30 minutes at M-pace, I just do most of that run at an easy pace and then do 5-15 minutes of M-pace running at the end.
Jonathan Savage goes as far as to recommend that during a taper all running should be at marathon pace. That can work, though I find that approach a bit restrictive. For this taper I started by running easy with a little bit of M-pace running early in the taper as volume was still close to my typical training. Then I increased M-pace running as the race draws closer and my volume tapers to its lowest.