Not that I am an expert, but let me deposit a few general ideas.

First--say February, March--you work your way through some base miles to gain some spinning souplesse and basic endurance (seems that you have already done that).

Then--say from April onwards--you begin to do different types of rides. You do a bit less of basic endurance work and more resistance and interval training (i.e. closer and over your anaerobic threshold). Thus you are gaining more pedalling power and will be able to be tougher on yourself. All other things being equal, that means you are becoming faster. Of course, you throw in a little recovery ride--and perhaps even day off the bike--here and there.

You often hear that shorter but more intense work-outs are more productive than longer, ‘lazier’ ones. Yet, I like my (weekly club) rides to be long (at least 100 km/62 miles). During group rides it is harder to stick to some schedule (slower vs. faster riders, different tolerance/pain thresholds, irregular intensity, etc.), but somehow I am convinced that the group rides have made me faster overall. In any case they often happen to be faster--and more stimulating--than my solo exploits.

It is difficult to go beyond these generalisations, since all our situations/abilities/etc. vary.

Much can be achieved on intuition, but I would say that a heart rate monitor, as well as some good reading, is needed to underpin and fill in your specific work-outs.

I don’t know the Armstrong Training Bible, but I would think that it contains some good clues as to the different forms of training and how to combine them. I have worked my way through some pro training literature (in Dutch, unfortunately) and learned more than I will ever need. But with a bit of fantasy, you can adapt the pro stuff to everyday life+level cycling.

Have fun!