There seems to be a small but vocal contingent in the running community that thinks participating in events like marathons and half marathons is only a worthwhile/respectable achievement if it is done at some arbitrary threshold of competence/excellence. I don't particularly understand this point of view and especially do not understand how combative and mean-spirited many of the proponents of this viewpoint become in attempting to belittle the achievements of others. Just some of the less favorable aspects of human psychology I guess.
Fortunately I have not seen much of this at rD events, although I see from other posts there have certainly been some unsavory moments even at Disney.
Back to your original post; I think most have covered it well. As long as you are aware of your surroundings you should be able to slow down without hindering others and raising an arm is always helpful. I run/walk with both an erratic interval (based on counting in my head to *hopefully* stay mentally sharp) and erratic pace (because I am terrible at pacing myself) and have not really had any major incidents. There are going to be 20,000+ runners out there. More than likely you are going to get annoyed with a few people and a few people will probably be annoyed with you (even though probably won't realize it). It's kind of par for the course with an event this size