I'm a mid distance guy. I mostly run the 800m, but I also run the 1500m fairly regularly and sometimes run the 3K indoor. Outside of a few local road races, I really never race longer than 3K.
Anyway, USATF XC Nationals was held near my school one year, so our coach decided that a few of us should run. Despite the fact that I'd never run an XC race before and that I'd never raced longer than 3K, I went ahead and ran the 12K XC race.
Being used to much shorter and faster races, I got out way too fast. I felt great through a quarter mile or so, but when I looked around and realized that I was right in the middle of the pack of guys who were actually trying to make the team for World XC, I knew I was in trouble. I tried to cool the pace a bit, but the damage was done. By the 4K mark, I was completely cooked. My pace fell way off and people were streaming by me on both sides. By 8K, pretty much everyone had passed me and I was running all by myself behind the whole field. The top few guys lapped me on their final lap (the race consisted of 6 2K loops).
My average pace ended up being about what I'd run for a distance run. When I looked at my splits, I basically ran a few low three kilometers, then immediately dropped to 4+ pace for the rest of the race. I usually don't get down over bad results, but that was the one race where I'm truly embarrassed to have the final time attached to my name.