I ran the St. George Marathon in October. It was my first. In training, I averaged right around 8 minute miles on my Saturday long runs. On actual race day, I was right on that pace until about mile 18 when the wheels kind of came off. I slowed way down (probably pushed it a little too hard in miles 7-14 (even though I felt pretty good)), started cramping, etc. I still finished under 4 hours (which was my "make or break" time), but the last 8 miles were not pretty. I think my overall pace was at 9:11 or something.
I felt like I had trained reasonably well, and while I would certainly slow some toward the end of my long runs, it was never as dramatic as what I experienced during the actual marathon. I also had never cramped up before. The first was certainly a learning experience. Looking forward to the next one.