As someone who went from being able to run 3 miles to completing a full marathon within 7 months time, I will say this, and you probably won't believe it, but it is true: when you are in the midst of a marathon training plan, and you are following it diligently, keeping up with both the total weekly mileage as well as the long runs, it truly is not any more difficult to complete 18 miles once you reach that point than it was to complete 8 miles the first time you did it. I was following a training plan that increased my long run by 2 miles every other week and dropped down in the intervening week. I, too, was frightening as heck each time I had to hit a new distance, but except for the last 2 miles or so, the physical task of running that distance did not require any extra effort at mile 16, for example, than it did at mile 10. You adapt and you get stronger.
My longest training runs were 18, 19, and 20, and I never hit "the wall" in training so I figured I was safe in the actual marathon until mile 20 and then all bets were off. I kept my pace, I drank and took gel at the proper times, so I was very surprised a little way's into mile 19 when I did hit the wall. I can describe exactly what it felt like to me. I had always imagined that exhaustion would come on gradually, but it wasn't like that at all. I was running along feeling strong when in an instant, I suddenly felt like my legs were lead. Scared the hell out of me. That was the moment when the race shifted from my legs to my head and heart. The world around me disappeared and I faded into my own mind telling myself how I "only" had about 10K left which I had done hundreds of times in training, and I was going to do this. Peripherally, as if in a fog, I saw people around me stopping to walk, and I kept telling myself "You didn't come here to walk" and "whatever you are suffering now will be worth it when you finish." I played word games and sang songs and did whatever I had to do to keep moving. Somewhere around mile 23 I guess one of the gels kicked in and I felt a tad stronger, and when I passed the 25 mile marker, I knew I was going to make it. But that Wall was a b*tch.
My strongest suggestion is that you buy the book The Non-Runners Guide to the Marathon by Dave Whitsett because it really trains your mind along with your body to handle the marathon. If I can do it, anyone can - at least, that is, anyone who really wants it.