For those who have run this before, do you listen to music/mp3 player while you run or is there enough going on you don't need to?
I'm in the minority, and I acknowledge that. I train with tunes, but I never race with them. Lots of reasons, but the main one is I like to interact with others. I like to offer and receive encouragement. But, if I run up on somebody with earphones in I assume they can't (or don't want to) hear me, so I just let them be. It kind of isolates you from the event.
Mile 4-8 and 10-11 can be a bit quiet, but I like to concentrate on my form, talk to others a bit, and pray, etc. I totally understand why people like music, but I try to encourage people not to isolate for a race like this because it kind of takes them out of the social part of the event.
Spectators also cheer you on and you would miss statements like "nice red shoes" or "way to go Robin Hood"

I'm somewhat fortunate because up toward the front, nobody wears headphones, so it isn't an issue.