The following story showed I obviously was fit enough to run the time, but at the time, I was nervous that I went out much too fast.
My 5th year in college I was the fittest I had ever been, and was entered in a 5K at Iowa State in January. At the time, I had a 15:04 personal best, and was hoping to break 15. I thought it was realistic, but still nervous as it was my first indoor 5K in 4 years, and first race in 3 months.
It went out hard and fast and after a couple laps I settled into a rhythm. I went out hard, but wasn't exactly sure where I was as Iowa State has a 300 meter indoor track. I passed the 1600 in 4:39, which was much too fast based on my personal best. At the time, I thought, well I might die, but I have already committed so I might as well stay smooth and strong and see what happens. Anyhow, I passed the 3200 in 9:22, and held on to run 14:45.
Now, I had obviously put in the consistent training to do this, but until you actually run the time, I had doubts about what I could do. It was real eyeopener and gave me the confidence to go on and run personal bests that spring from the mile through the 10K.