The term comes from literature; an epic is a type of heroic story. So it should be a ride of heroic proportions. How is that defined? Distance is a big part of it, but part of the classic definition of heroism is overcoming of obstacles. So I don’t think a long ride in itself qualifies as epic; there should be at least one moment where it’s not clear whether the thing will be possible, that the hero pushes through and conquers.
Some truly epic rides:
captainwelch’s Dalton Highway ride: