Originally Posted by Reynolds

I'd take a different approach than most here. What worked for me and many others was to build a strong base, by riding many hours a day, many days a week, not too fast and with few stops, for a long time. This will condition the body to endure long rides and do better use of its reserves. Speed will increase "naturally" - not competition level speed, but faster than before. Intervals and very hard efforts can be added afterwards to reach a given rider's potential.
Of course you need lots of time to do this, it's not for everyone.