The HM starts at 11:00 this year. This means that the entire HM field should merge with the 8:00 to 10:00 min/mile runners at the Marina (2M/15M). Things may be a little 'tight' for approx. 1/3rd mile, until you move away from the river. After that things should open out, but the third relay changeover point (3.5M/16.5M) may also be tight depending on the crowd (and stewards). After that everywhere is wide enough to cater for everyone.
With the earlier HM start, the walkway after the Mardyke (12M/25M) shouldn't be too big a problem, but the HM leaders will be passing 7:00/8:00 min/mile Marathon people here, but they should be relatively well spread out.
Last I heard, there were approx. 1650 entries in the full, 1850 in the HM and 450 relay teams.
Finally, FFS everyone, Run the SPR line!!! (Shortest Possible Route). Every year I see people 'glued' to the sides of the road.
Addendum: I forgot the start area - The start is appro.x 11.5 miles into the Marathon, so for the first 0.5 miles of the HM you will be running with (hopefully) a relatively small number of 10:00/11:00 min milers. If any of the Marathon people get caught up in the HM after the split at the Castle, please re-direct them back to Marathon route. Guaranteed there will be a few.