I did this in 2007. I had been running decent mileage, like 70 mpw, but mostly doubles. Not training for anything. Wife was pregnant and I was focused on work. I was 27. I jumped into a pretty major marathon with a co-worker's transferred bib. You used to be able to do this easier than you can now. It was legal; I was not a bandit. We did it at the expo, not online.
Anyway, I wanted to break 2:30 because in 2004 I had run 2:30:3X. I ran the first 18 miles on pace and felt amazing. At 21 I had to stop and go to the bathroom in a port-a-john (#2). I had never done that before in a race, but there was no other option. I was very efficient in the crapper but was now off pace... but still felt okay and was trying to get back up to the small pack that I'd been running with all along. Made it back and was like 20 seconds off pace at the 23 mile mark.
Not joking at all. My legs started cramping around 25 miles and they would not stop. This has never happened to me on a run or race ever before or after. I literally could not finish the race. It was like that video of the two guys (ladies?) trying to finish the ironman by crawling across the line. Instead, I just sat down and tried to let it pass. Finally got back up and crossed the line in the high 2:40s. Terrible last mile. Physical agony.