There are actual guidelines from an official organization. I don't remember who posted it or from what org, but something about doing a minimum of 10-15km/h.

Personally, I think if you do 30km/h, but take an hour nap in the middle, that's not a century.

VR, you should still be very impressed with your self since, IMO, it's easier to do 100miles in a day, than do 60, wake up and do 60 more.