KSH,
One way to get faster is to ride with someone, or with a group, that rides a bit faster than you. You'll push yourself just that extra to keep up. If they are TOO fast, it won't do any good 'cause you'll burn out to quickly. But you can hang with a slightly faster rider(s) for a good long time. You may be tired when you are done, so give yourself an easy ride the next day for recovery.
I agree about intervals. They are NOT fun to do - they hurt, but they will improve your speed. You can do them on the flats. Warm up well before you begin. Ride as hard as you can for a set amount of time, then totally recover and do it again. Start with a couple of intervals per ride and increase as you become stronger. Or you can think of hills as your intervals (depending on the length of the hill. Too long won't cut it.) I have lots of short, but steep hills where I live and I will weekly choose a route that takes me on the worst hills and try to go up as fast as possible. Recover between hills. Then take an easy day for rest.
There are many good books available now that cover all aspects of training on the bike. You can do a google search. Then choose one that suits your needs.
Riding consistently at 20+ mph is really moving! Once you get there, are you considering racing?
annie
Time is a companion that goes with us on a journey. It reminds us to cherish each moment, because it will never come again. What we leave behind is not as important as how we have lived." Captain Jean Luc Picard