In college I was on a track and field team doing 1,600 and 800 meter events. I was number one on my university team setting all time best scores in those events. One day we had a city wide marathon event and I decided to run 10k thinking that if I stay behind a group of guys doing a full marathon I would maintain a good pace and run a good 10k race. I was really wrong, at about 8k it felt like my legs gave out and I barely finished by walking :) I guess you have to train for things like this :)