It is the idea that any serious runner is by definition a world class athlete. What about regular runners who are not world class athletes, are they serious or not?
Apparently not
I'd consider myself a fairly serious runner - up to 2,000 miles a year and there's no way on Earth I'd be able to run a sub 6-minute mile, much less a sub-4.
That's because I train for distance and while I can run 20 7:45 minute miles anything much quicker gets difficult very quickly.
If I had to run 400 metres flat out, I'd be useless for a while afterwards