In order to get faster, I would suggest incorporating some intervals into your workout. That will help you maximize the time that you have. You might search for "intervals" here to get some specific suggestions. Beyond that, if you can hook up with some riders that are faster than you, that will help. In order to get faster, you have to be willing to push yourself out of your comfort zone for longer and longer stretches. For me, that entailed going to weeknight training rides and trying my best to stay with the group. At first, I was hardly ever successful, but it got easier and easier. Of course, now I'm slow again. The other thing about getting faster is that you have to keep up with the training.

The other thing I have to wonder is whether you've learned to "spin" in the right gear. Ideally, you want to choose a gear (at least on the flats) that allows you to turn your pedals at a relatively fast pace, somewhere around 90 revolutions per minute. While not all cyclists are alike, an approximation provides the best balance between leg strength and aerobic capacity. You might think about--if you don't have one already--investing in a bike computer that offers a cadence function.