Originally Posted by CommuterRun

Improvement comes in rapid gains and plateaus. Sooner or later you'll reach a point where you're seeing no improvement at all. Train through that, and you'll see another gain, followed by another plateau, etc. You'll also see more and better improvement if you mix up your training, i.e. intervals, easy, spins, hammer, easy, hills, etc. Throw some upper body in there too.

As for myself, I have seen my performance fall and I get progressively slower if I try to hammer all the time. Which is one reason why I llike to mix up riding different types of bikes and different bike/trailer combinations.