When I took basic training in the Army in our company was a group of National Reservists from... geez, I forget... I think Wisconsin? And all on the University swim team.
When it came to the 'endurance' runs, during our training, none of us could keep up with them... not even close. I won't try to repeat it here but they told me about their training and it was surreal... and most of it NOT in a pool.
I don't know if any other sport requires such intense training... maybe cross country skiing? - JamesJM