It seems to be true. As time goes on, I find that X amount of effort still feels like X amount of effort; but I'm going faster. Well, nothing's wrong with that.
I want to be able to cruise along fast enough to complete a century in under 8 hours (total time). Allowing for rest breaks along the way, I'd need to be able to average almost 13.5 mph, maybe more. Considering that the century I'm aiming for has some hills, I need to be faster than that on the flats, and develop more leg strength for climbing. Hmm. Do you think it would help if I slipped into that polka-dot jersey? ;)