my first post was kind of mean, sorry But it's the truth- the only way to get stronger is to ride. If you're really interested in training properly, there are books (most notably "the cyclist's training bible") which will give you a good background on the theory behind cycle training. In reality, though, it comes down to putting down miles.

Also, if you don't already, you might look into getting some clipless pedals or at least toe cages. They will allow you to use a wider range of muscles during the pedal stroke, so you don't isolate and destroy the quads so much.