In 1997 I was running a lot of short races and was a 16:30 5k guy. I had never even considered running a marathon. A buddy asked me to pace him at Chicago to try and break 3-hours. I asked him what pace that was and he said 6:47. Ha! that's my easy day pace (I was running 35-40 miles per week, most of it balls out - but I didn't know better).
So we made a deal, at 20 miles, no matter how we felt, whichever was feeling better was given the clearance to take off (I, of course, assumed this would be me). I did a 16-mile run in prep of the marathon and it was the longest run I had ever done up to race day.
We ran together till 20 miles and agreed we still felt OK and would stay together. We were slightly ahead of the 3-hour pace group. At 22 miles, I knew something was up and told my friend as the 3-hour group was beginning to encircle us. "I'm gonna back it down, you should stay with the big group." I went from clicking off 6:45's to 11 min miles in no time. My legs were cement. I hobbled in with a time of 3:09:5x and was more sore than I have ever been in my life.
My buddy ran 3:03 as he faded from the three hour group too.
The marathon is a different animal. It was a few years before I would try again but this time I trained for the race correctly. AS a predominantly fast-twitch type runner, the marathon was never going to be my best race. I ran a couple marathons and the third official race ran 2:45. That is my lifetime PR. All of my short distance PR's said I should have run around 2:33.