I'm slow, and can plod along forever. Certainly there would be something with not burning oneself out too quickly.
A riding buddy can help with motivation.
On the other hand, with the longer rides, one can run into a crunch of losing daylight, or not enough hours in the day.
So, say one rides 10 MPH for 200 miles, that ends up being a 20 hour day... and a very long day in the saddle. Bump it up to 12 MPH, and one is down to 16.7 hours. At 15 MPH, one is down to 13.3 hours.