Perceived effort doesn't change. If you push hard, you might be slower than before, but working just as much. So if you like to suffer with your cycling, don't worry, you can always suffer.

Take a look at this New York Times piece: http://tiny.cc/lo7rlx. You'll see that you might be able to stay fit and reasonably fast for far longer than you might have imagined. Actually, you can skip the article, because it basically states that people who keep exercising throughout their lives stay reasonably strong, even into their 80s.