Riding a long distance for 24 hours is 80% mental. The other 50% is physical.
Lots of online blogs say that it is not necessary to ride long distances to prepare for a 24 hour event. I say hogwash. You need the confidence of riding a pretty long distance to overcome some challenges and to develop the confidence and mental toughness. It is true that experienced long distance riders do not need much more than 6 or maybe 10 hour training rides to be able to do a solid effort on a 24 hour ride but only because they know how to handle the suck.