The Leg Extension, like the Hamstring Curl, is a nice exercise that helps increase your cadence as they build the muscles in your legs that allow you to twirl the pedals more quickly.
The leg extension is performed on a bench with a pulley system that allows you to raise a stack of weights. You sit on the bench with your feet pressed against a bar that, when it is raised by slowly kicking your feet out and forward, lifts the weights into the air.
In these exercises, do three sets with ten repetitions in each. The weight can be the same in each, but the rest interval between sets should be only 30-60 seconds. Start light -- maybe just 20% of your body weight at first -- until you find the right weight for you for these exercises. The right weight is one that challenges you over ten repetitions, but that you can ultimately complete.