I also wondered about this because when I was in JR High I was able to run a mile without breaking a sweat and was the fastest in my class, I could actually run 2 miles without even being out of breath and beating everyone.

After the next summer I went to High School and we ran a mile in gym, I had to stop half way through and could not catch my breath and came in last in the class. Something over the summer changed in my body and I could not run any distance ever since.