A friend is the strongest 50something guy I know in this area, routinely averages 20+ mph on his typical 20-40 mile rides on the same routes where I struggle to average 16 mph. On a good day I can hang with him for up to 10 miles. But I've never seen him hold a 20 mph average over a full century.
That might be because he adjusts his pace to suit the folks he rides with. He's much stronger than I am but when we've ridden together he doesn't keep pushing the pace and doesn't mind holding back.
But I'd still be surprised if he could hold a 20 mph average over a full century. I follow some local pros and strong amateurs and they usually average 18+ mph over 100+ mile rides.
Probably depends on the terrain and wind. We don't have any mountains, and few continuous 2% grades as long as a mile without flattening out. But there's a lot of wind and our roller coaster terrain has lots of double digit short steep climbettes. It's tough to gauge an effort for a long ride, especially when the temperature gets to 90F and higher.
Neck and shoulder spasms from injuries usually stop me around 50-75 miles. The few full centuries I've finished the past few years, my average has ranged from 12-15 mph. And that's moving time. I need lots of short breaks to stretch and massage those pesky neck muscles. I usually aim to finish within 12 hours, including rest breaks.