The problem with training, be it for any sport, in this case bicycling, is to get the balance right. if you're a weekend warrior then it's not quite as important as it is for a professional cyclist, still you can over do it and get ill or injured just before the peak you were aiming for. I just posted a poll on Vets training to see what most people do and was surprised that most had similar training volume, there are the odd exceptions. If I was to take an average it would be 11 hrs per week though nine out of the 16 posts trained for 8 hrs a week, myself included.
Volume v Intensity … It is well known you can work hard or you can work long, you can't do both, well of course you can but you miss the point if you do. The most extreme example is what you can do for a minute all out effort is very different then what you can do on a bike tour over nine consecutive days. The problem with volume is you need time to do the work load. The problem with intensity is you need the rest to recover from the training and also the freshness to be able to perform at that high intensity.
Specificity … It is known, maybe not as well as it should be, that muscles are very dumb ! they lean one thing and are unable to make anything else from that. If you what them to know two or three things you have to teach (train) them that too. They will never figure it out for themselves. So the whole concept of specificity is based on this. Your fitness is very action, activity, velocity specific … action ( what your ride), activity (over what terrain), velocity (at a cadence of). Specificity demands very specific variations, bit of a paradox, huh ?
Recovery … is what is needed so your body can adjust-mend-strengthen-adapt to the new and ever increasing demand. if you are able to back up what you do day after day. Then you need to ask yourself " Do I need an increasing adaptation demand in my training ?"
In a perfect world … I would train (bicycle, all the other stuff, weights etc not relevant) 12 hours a week on the bike, However I choose to sacrifice training volume for intensity. Because I can recover at work, sitting at a desk writing reports.