Swimmers' training volume is something regular people could never keep up with

‌I looked up how these athletes train,
and apparently they do 2–3km of just kicking as a warm-up. I tried to follow suit during a freestyle session,
and my legs completely gave out. Doing 2–3km of actual swimming is tough, but I can manage it; however, doing it with just kicking is on another level.
Setting breathing aside, there came a point where my legs just lost all strength.
And they say they do their main training even harder after that..
They really are professional athletes, wow...
Seeing things like this makes me realize how truly amazing marathon runners who complete 42.195km are..