One thing that I have not seen taken into account on this topic is efficiency. I can take two months off the bike and burn a lot less calories riding 40 miles at 20 mph than I could have two years ago, the reason being my riding is more efficient. I pedal full circles, I know when to push myself and I know when to take it easy to bring in a good average speed a whole lot better than I did when I was a beginner-I can get into a more aero position saving even more energy.