What I did when training for triathlons was to get a workout program from a professional trainer. These are relatively inexpensive if you buy it on-line. Stick to the program and try very hard not to miss any workouts. Pay attention to what they say about the workouts. If it says to spin and keep the heart rate down, do it. Don't deviate from it. When I went out on fun weekend rides I tended to exceed my heart rate and paid for it. When I did my tri's though I almost always was in the top 5 in my age division on the bike and a lot of times I'd have the best bike time for my age group. (Xterra's I took first, they don't train for the technical part and get freeked by the rocks). I guess it all depends on what you're trying to accomplish.
My coach taught me there are basically two types of training.
Endurance = total time on bike mostly concentrating on pace. These workouts were a lot different then the speed ones.
Speed training was done with intervals and more recovery days. Very painful but you'll get a lot faster.