Just a speculation or guess on my part, but they are likely to assign you to the last corral. You'll know more definitively in about 1.5 weeks when the waivers/corrals are released.
Now, don't freak out. When you go to the expo to pick up your bib, head over to race relations. Bring with you the 5k and recent HM results printed out. Have those sheets to help show what you're capable of at those distances. Bring a smile and hope for the best that they may be able to move you into a further ahead corral. They might be able to help you, and sometimes they aren't. It's no guarantee.
But, even if you do end up in the last corral unable to move up, just keep some things in mind to help your race experience. Show up early on marathon morning. Head to the front of the last corral so that you can be one of the first people released in it. Stay calm and run your race as best you can. Stay courteous to those around you as it may get crowded and the people around you may or may not be running as fast as you are. Try to keep a positive outlook. Don't get frustrated. There are definitely sections of the course that open up more than others (regardless if you're in corral C or P). So don't worry if a certain section seems overly crowded. Just conserve energy, make a small move when you can, and then bide time until the next wide open highway section. You'll waste lots more precious energy by weaving than just sticking behind someone for a few extra seconds. Being in the last corral won't completely inhibit you from running a fast race (if that's your desire). Below is a screen shot of some random marathon results:
View attachment 287187
The left column is "Clock Time" and the right is "Net Time". The differential between them comes from how long after the race started that they actually started. So a person with a 35 min differential started the race 35 min after the first person started (therefore they're in a further back corral). The last corral leaves about 50-60 minutes after the race starts. Just from this small subset I see a few people who ran sub-4 marathons who started in one of the last corrals (4:37 vs 3:52, 4:29, vs 3:53, 4:31 vs 3:54, 4:54 vs 3:56, and 4:51 vs 3:56). And this is just a small subset that I barely even had to try and to search out for these results.