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)