It's even simpler than that. The power necessary to maintain a speed on the flat increases as the cube of the speed. IOW, one has to work very much harder just to get 1-2 mph faster on the flat, and the percentage increase is small. Hills, however, are another story. Hills are pretty much linear when ridden at the speeds most folks ride them, 6-10 mph. Not much wind resistance down there. So your increased effort goes directly into cutting down your time, rather than into entropy. IOW, ride hills faster. That's how one gets their average speed up. Riding hills faster is a very effective training method for riding hills faster. Works every time. Not only that, but being able to ride hills faster will also make you faster on the flat. Top climbers are often top TT competitors.