Ohh I like your approach as well! I think that changing the impact of the roll is an interesting solution!
For me, a regular challenge is something a hero is roughly equally matched with (two equally skilled warriors fighting),
A tough challenge means they are a little outmatched (a human warrior competes with an elven archer at target practice)
An epic challenge means the hero is not favored at all, but it is still possible ( a warrior trying to arm wrestle an ogre)