Being comfortable with that distance and more, is key. The physical plays into it, but the mental is there too. So many folks say they don't like to ride that far due to boredom, hitting a wall at 70 miles, stuff like that. Heck, I'll look at a ride 30 miles in and realize I have how much more to go?
Can’t remember the last time I intentionally rode a century. Had one forced on me in 2009 while touring in MT. It was either do it or not get to ride Going to the Sun Road in Glacier the the next day, which was to be the highlight of the trip. After three climbs, including one of 12 miles, and some rolling terrain I made the mistake of looking at a mile marker and doing quick math. Realized we still had 54 miles to go. After that, I would purposefully look away when I noticed a marker up the road. At the end of the ride the GF couldn’t figure out why there were urinals in the women’s room at our campground until she realized her mistake.