[dyauspitr]

A mid tier marathon runner can do a mile every 20 mins, not a kilometer. It’s a significant difference.

[mb7733]

This is besides the point but a 20 minute mile is a typical walking pace, not anywhere near a mid tier marathon pace (regardless of the definition of mid tier). That would be an 8:40 marathon.

[sandworm101]

I meant the effort required for the pace, not the literal speed. For a skilled swimmer, 20min per km can be maintained for a few hours, like a runner maintains marathon pace for a few hours.