Some advice from the I ran a marathon despite not being a marathon runner and sort of kinda hating to run camp:
-band-aid up those nipples and be sure to Vaseline the family jewels.
-Don't be afraid to walk for a minute now and again, it will save you some energy towards the end
-Whatever anybody else says, if you can run 20 miles, you can run 26.2.
-The mind is your most important tool. At one point during the marathon you will feel awful, but if you can rationalize that it will not get any worse than that moment, and that all you have to do is live with the pain and misery for a few more miles, it will do wonders for your spirit, and you'll actually pick up your pace.
-I am living proof that you can actually will yourself through a marathon, and if i can do that, you'll be a rock star.
-Not everyone is a "group" or partner runner. I personally hate having to tell people "you're doing great Bill, keep it up." And don't me started on them trying to pump me up. Please, i'd rather just focus on something else and pretend i'm not running.
-That last mile or so may end up being one the best times of your life, so enjoy it.