Yellow's right; to increase speed you have to do interval training. There are alot of good articles on the internet, or written by professional cyclist etc. Basically, its a maximum - minimum concept. Find a course that has varied terrain. And start out, warm up, ride at a good steady pace and once comfortable, do an all out sprint, pushing up the gear and cadence. Do this from a short time (say 30 seconds) only, back off, catch your breath, repeat. Each time you do your ride, increase the interval time. Start sprinting up short hills, etc. Basically it takes leg stength to sprint and ride fast. Thats why we have cyclist who specialize in certain areas; the climbers are light and have long lean muscle when sprinters are heavier and have short, explosive muscles. CG folks are in between.

Good luck!