You're right, what I should say is per continuous riding session - I used to take more like 38 hours for 600s (just under the cutoff, haha). There are breaks, usually 30m max, but I tended to average more like 18kph over complete ride. I wanted to reach 20kph, but had a kid instead – that kind of threw a stick in taking long rides.
Here's more info if you're curious, it's a fun sport called randonneuring: https://en.wikipedia.org/wiki/Randonneuring
Here's an example 400km route from an area near me run by the BC Randonneurs: https://ridewithgps.com/routes/33536016
It's a great way to get to know areas better. Very meditative, challenging, and rewarding.