I am probably not the best example since I am not natural long distance type. The best way to explain it is to break it down in to phases.
1. The first couple of miles (3-5) felt uncomfortably slow, sort of like doing a recovery run when you feel like running hard.
2. Mile 5 to about the half way point felt really good, soft of like a basic training run.
3. Mile 13-18 is when I started to focus and had to work to stay on pace. Sort of like the last couple of miles of a long training run.
4. Mile 18-22 is when the realization hits you that you are in a long a$$ race and for me this when I start to struggle. By mile 22, I am racing, focusing on my goal, trying to get to the finish like just like in any other distance, but in this case I have an entire 5K to go.
5. Mile 22-24 in all but one or two of marathons, I am on running on fumes, my runner's gate has switch to a shuffle, I am in pain and just trying to hold it together.
6. in the last half mile or so I am still in pain, but I can hear the crowds and know the is near finish, I feel the excitement and I'm usually able to move at a slightly faster shuffle.
That's pretty much it, I have only had one, maybe two marathons were I felt like an athlete/competitor at the end, by body usually breaks down around mile 22 and it is a slow painful slog to get to the finish line...the pain is pretty bad. It's not like one hard punch to the head like the intensity of finishing an 800m, but more like a dozen punches to the gut. The best marathon I ever ran was when I was unregistered and was supposed to be pacing a friend to the half and then to 18 miles and decided to keep going. This is the experience of a late-30s family man trying to run sub-3 off of 50mph with most of my long runs between 10-15 miles and one 20 miler.