Back in the 1980's I lived next to a park that had a wonderful 5K loop that many people used. One morning I was out running my usual 5k, on track for close to a personal best when someone passed me so quickly and quietly that it startled me. There was a noticeable breeze from her passing. She lapped me twice in the course of my 5K (meaning she did close to 15K).

I found out later she was the current world record holder at 10K meters. And she wasn't pushing it that day.

These folks can run like you wouldn't believe.