Last time I did a 20 minute MMP test, I was pretty done at 18 minutes, then the road went uphill at about 18:30 and my power went up with it. I don't know if hills just keep us honest, if the different position recruits more muscles, or if somebody shot me with a tranquilizer dart but filled it will speed instead. But I've heard a lot of people say hills draw more power out of them at the same feeling of effort.