In truth there is no set plan that everyone has to follow and the taper is very much a personal preference as to how long it should be and what you should do during it. Obviously you are going to be doing less running in order to be 'fresh' on race day, so how you approach this depends on how many days you normally run when you're not tapering.
Taper about a week before
With the half marathon I would think only a week before the race you need to start thinking about the taper, whereas with the marathon you could extend this to 10-14 days out or even more because it is double the distance after all.
During the taper you want to be reducing the amount you run by about 50-60 per cent.
During the taper you want to be reducing the amount you run by about 50-60 per cent. Again, this can vary depending on how you feel about having complete days off running compared to doing a little jog. Personally I find that having a day off doesn't agree with my legs as I feel stiff the following day. This is probably because I am so used to running every day that my body goes into shock when I don't! So you will need to experiment a little bit and find out what works well for you. Taking a day off running 2 days before the race and then doing a small jog with some strides the day before might work well for me, but you may want a complete rest day before the race. Like I said, it is personal choice and only you are going to know how your legs feel after a rest day, compared to a day of easy running.
Run at race pace over shorter distance
I always find that running at the pace you want to race at for a much shorter distance is helpful in the taper. This way you can tune into the pace you are going to need to run and get a sense of how that feels. Normally this would just be for a minute or two and with plenty of recovery because you are not going to get any fitter race week. It is all about getting the legs ready. One thing an old coach of mine used to say was; “The hay’s in the barn!” Basically he meant all the work had already been done so it’s too late to do anymore!
Avoid too much in final week
The biggest mistake people can make is doing too much the week of the race. They start to panic that the race is finally upon them and try to cram a month of training into a week. That is a BAD IDEA. If you haven't done the work by now then you'll have to accept that and train better the next time round. It is simply too late now. See what you can do on very fresh legs.
It is always better to go into a race under prepared rather than being over trained and tired on the start line.
It is always better to go into a race under prepared rather than being over trained and tired on the start line. Training should be hard but not so hard that you leave all your best running in training. You want to race well and have a time at the end that reflects all the hard work you’ve put in.
The taper can be perfected over the course of a number of races until you find out exactly what works for you. Just don't keep running the miles and doing sessions the same week as the race and then wonder why it has all gone wrong. If you think it's too much, it probably is. Good luck, race well, and get those PBs!