Most people get faster by just riding more - up to a point. Whether you've reached that point or not, I don't know.
An organized plan for your training will certainly help. Adding some spin classes to get your legs moving faster might help. I also wouldn't discount group rides. Ask around at the shops, look on the internet, ask at the clubs ... chances are there are some group rides around that go at slower paces and focus either on beginners or people interest in a relaxed, social ride.