Intervals! Intervals! Intervals!

There are a lot of ways out there to do them, and some excellent online resources about training (especially for race events) that incorporate them. Basically, they are periods of intensity -- higher speed, higher cadence, higher watt ouput -- interspersed with short periods of recovery. They might not feel so good at the time, and the rest never quite feels long enough, but after a few weeks of them you will notice your baseline speed has increased a fair bit. With continued work, you build a base of fitness and speed that's much more structured and noticeable than if you simply ride for long stretches without increasing your aerobic and mitochondial capacity. When I began doing intervals, my coach had me doing 2 10-minute intervals with a 5 minute rest during a 1 hour workout; now I'm doing 2 20-minute tempo intervals (even higher on the exertion scale) with a 5 minute rest during a 90 minute workout and some fast-cadence exercises afterwards. The rest is for endurance and putting the miles in.

Can't endorse intervals highly enough.