I have said it many times on this blog and in person on rides with friends when talking about training, "make your easy rides easier and your hard rides harder."
I have followed Dr. Seiler's expert advice for a few seasons now and his three zone model. Try to see if the concepts he espouses in this TED Talk make sense to you, too. The best athletes work with great focus, and that is not by wasting time.
Give this a listen and then go research some more on his work. I think you will find it thought provoking and amazing when put into practice.
This page has a great interview with Seiler and the TP guys. It also has links to his Twitter and other places you can go learn more about his wise ways.