Here's an idea. Go find a steep-ass hill. So steep that it shouldn't even be paved. Pick a gear that will have you at 30rpm going up it with a hard effort. Clip in with normal cleats/clipless pedals, and climb the hill at 5x interval pace (like you could just barely do that pace 5 times). Ride back down and change into running shoes. See if you can match your previous time, with any level of effort.

(Hint: you can't)