A reduction in volume and an increase in intensity increases the stress on the CNS and lowers the possibility of performances. The taper consists of a slight reduction of intensity on the speed work and a reduced speed volume by session and by week. The slower work has no real cost to the CNS and needn't be reduced much.
Leading into the marathon it is a good idea to do one last hard workout on the Saturday of either long intervals or a short simulation run and then take three days easy to recover and prepare for one final tune up workout on the Wednesday at goal pace or slightly faster with long recoveries and then take another three days easy to prepare for your upcoming marathon.
The following would be a good taper protocol for a Sunday marathon;
- Saturday - 6 x 8min with 2min recovery
- Sunday - 60min easy
- Monday - 60min easy
- Wednesday 6 x 3min with 6min recovery
- Thursday - 30min easy
- Friday - 30min easy
- Saturday - 30min easy
- Sunday - race
These are long rest periods on Wednesday but they allow you to dial in pace without stressing the CNS and fatiguing the body for your big event.
The other thing that is important when tapering is taking more time to rest, naps, to bed earlier, reduce caffeine and other things that stress your adrenals, reduce time on computer and TV and truly rest so that your body can actually rest - as the training objectives have already been met, and you are now merely being maintained with as little effort as possible to allow the organism to rebound to super compensate for the "Big Day".