I have been encouraging everyone to do their longest run (whether it's the full HM distance or not) about two weeks to 10 days before race day. It’s generally accepted that it takes 10 days to 4 weeks to benefit from a run so running further in the final 10 days will only make you more tired before race day without providing any benefit. Undertrained but fresh on race day is better.
So what are we meant to do in the final weeks before race day? I've had a quick look at articles on the internet and my main piece of wisdom is that tapering is an art, rather than an exact science, because it can vary so much from person to person. Here are some thoughts.
Unless you've got a niggle, don't change your weekly routine (eg how many runs) too much because this will confuse your body - it's volume we want to reduce so that you have a chance to recover from all the training
Don’t do any workouts that are going to leave you feeling sore. Rather than having your body trying to get back to normal health, repairing damaged muscle, you want it recovering beyond your previous strength levels.
Reduce your overall weekly volume, but by no more than 25-30 percent (this presumably is why the suggestion for the penultimate week is to do 10miles) as the body may otherwise feel flat
Often a taper comes after some of the heavier training loads building up to a goal race, and this can lead to immunosuppression and we see many getting taperitis or illness during the tapering phase, then blaming the taper for the onset of such illness or injury (Yes, the rest made you injured).
Record what you have done as part of your record of your training so that you know what works or doesn't work for you for next time.