A few more comments based on what has been said.
First, Zeke is right. Make sure your mile markers are right and get the course certified. I don’t know why anyone would put all the work into preparing for a marathon and then not choose a certified one.
Second, I alluded to something that Zeke very specifically pointed out. The course must be well marked and the people guiding the runners must know where the race goes. If you sense any possibility of uncertainty, remove it. I can tell you there’s no worse feeling than leading a race and wondering if you’re going the right way. I would be very upset if I was led off course in a marathon. I would be very vocal against that marathon probably for the rest of my life. When one works so hard to prepare for a marathon, it is completely unacceptable for their race to be ruined by getting led off course.
Third, something you stated. Your course is difficult. It does not sound like a race that people looking for a PR would go to. Be up front about this. Advertise it as a challenge. You won’t get runners who are looking for PRs but you will still get runners. Everything else still applies, though. No matter how difficult the course that I’m running on is, I want everything to be handled in a way that allows me to have my best performance. If I’m handing over my hard earned money for a race, that is my expectation. I highly doubt I’m alone on this.