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?
Oh, definitely. I'm comfortable in my own head when riding, though, and I actually *need* that time to myself, at times, so no worries there. At least that's true at the kinds of distances that we're talking about; I think that I've done six centuries so far this year, four of them solo and very necessary.