normally after a 1/2, I take a day or two off, but am back to my normal training routine, bar injury, within a week. I usually do one easy month a year, when I half my weekly mileage - normally November, and don't run at all for a couple of weeks in the summer.
good luck in the GNR, enjoy it and take regard of what your body says afterwards - you might find that none of the advise here applies to you!