Audax UK, cus that’s what I do, has minimum speeds for distance.
A BP can be as low as 10kmh if the organiser regularly has a lot of juniors attending, but organisers usually ask for 14.3 kmh.
Here is their regulation.
The minimum speeds may not be altered and shall be:
for events up to 699km - within the range 14.3 to 15kph;
for events from 700 to 1299 km - 13.3kph;
for events from 1300 to 1899 km - 12kph;
for events from 1900 to 2499 km - 10kph; and
for events over 2499 km, 200 km per day.
As you can see, AUK reduce the minimum speeds as the ride gets longer. They know about 'fatigue'.
Newbie and novice cyclists suffer fatigue at much lower distances than 699km no matter how much they eat and drink.
When someone posts "I average 17 mph", that's probably over a short distance of 100km or so. Will they average 17 mph over a 300km Rando?
Clever gits have spreadsheets that predict speed vs distance based upon a graph of Watts vs Duration ( minutes ).
It helps them 'pace' themselves on a ride of 200, 300 and 400 km that will last 10 hrs, 17hrs and 24 hrs.
When the distance gets to lower than 10 miles, the limit is aerobic capacity and lactate tolerance, and of course traffic. Don't go attempting Personal Bests on the public highway. You might be riding indirectly to A&E.