Laureate, If I were you I would run the half at slower than MP. Possibly setting off easy, and making it progressive. By then you will have some idea of HRmar.
So my plan would be to run:
4M easy, 4M steady, 5M at HRmar (86-88%).
The other alternative would be to "practice" the first half of your marathon. ie. Run it at your planned HR as per the first half of the marathon. For me this would be starting at HRmar - 10 (about 83%), and then letting it drift gradually up to HRmar - 5 (85-86%) at 6miles, and then finishing at HRmar (87-88%). But this may impact on your marathon as it is only 3 weeks before. Guess it will depend on your ability to recover.
I know I want to average 164-165 in the marathon, so I set off at 155, and aim to hit 165 at h/w. Hit 170 at 20, and then if the legs are in reasonable shape, 175 with about 5k to go. Throw it all on to the fire for the last 3 miles. My max is 187ish.