Alpha... I have run many a marathon. Let me give you one word of advice. VASOLINE. I know you have heard this before, but when I say put it everywhere, I mean literally, or you will start to bleed in places you didn't know you had. Good luck, you will love it. LOVE IT.
And remember, more people drop out at 20 miles than at any other point. So, even if you have talked yourself out of finishing (which you won't) make yourself go to mile 21. It looks a lot less daunting at 21 than 20. It is strange, but true. As they say, "20 miles is the halfway point of the marathon"...
It will be so fun, you won't even know you are hurting
Seriously, the first 15 miles will pass b4 you know it.