I would say the level of difficulty on a personal level would define how epic a ride is. If an overweight fat bastard who hasn't walked further than the car get's up and rides a bike up a hill or a few miles down the road, that is a considerable effort, and worthy of being epic. To that person of course.

My last day in mesa, I did a 50 mile trail ride. It was over 4 hours of riding. For me, that was epic.