This is a common occurrence in runDisney races. Experiences vary across the spectrum, but due to the very nature of runDisney races (where runners are going for PRs, stopping for pics, getting a beer, riding rides, etc.) it causes a greater mix of paces at any one point in time. Add in that in some places these races with 16000-26000 runners are running on small pathways/sidewalks (in addition to roads/highways) it means you'll get some bottlenecks and congestion. Looking ahead, the HM has a few notorious choke points that you should mentally prepare yourself for:
View attachment 636716
Around Mile 1, you'll go down to a single lane uphill. This gets congested.
The turn at Mile 1.5 tends to be fairly crowded. Even for runners like myself who finish in the Top 2%, start at the front of "A" corral (or now s1), that particular turn tends to be congested. Be patient. As soon as the turn ends, you end up on a highway that is several lanes across and it opens up quite a bit more.
Around Mile 3.5, you'll enter the TTC area and there's a brief moment where you go underneath the road. It can get tight here.
The MK Castle can be congested based on when you arrive there.
Mile 6 to 8 is known as cone alley. All the runners go down to a single lane. So it can get congested there.
After Mile 8, the course opens up quite a bit and there's a lot less choke points until you're inside EPCOT itself.
View attachment 636721
The start of the M going through EPCOT tends to be pretty congested.
Then similar areas as the HM through TTC, MK, and cone alley.
There's some congestion in the AK area, but it's not as bad as other places.
Blizzard Beach probably won't be that bad since it's in the parking lot.
DHS will be tight going through that single lane exit.
The DHS/Boardwalk path tends to be fairly congested.
***
Hope that helps with some other places to mentally prepare yourself for congestion during the HM/M.