Human limits are arbitrary.

There are many levels of capability, most people are not aware of.
When you think you can’t, you probably can.
When I use to play racquetball there were people I could easily defeat, and those who could easily defeat me. None of which were considered professional.

There are many levels of capability. It’s a long journey to mastery.

Ultra Marathon, etc.
Nature vs. nurture. It’s almost always nurture. With few limits on nature.
Future genetic engineering and hormone therapies change the game.
How Steroid use is a cheat.
every ultra triathlon (Ironman) event must involve more than 2.4 miles (3.86 km) of swimming, 112 miles (180.25 km) of cycling, and a full marathon (26.2 mi or 42.2 km) of running.