See Carmichael's new book: The Time Crunched Cyclist. He says that 10 hours/week is the lower limit of effective traditional training. Below that, you do better to do 6 hours spread over only 4 days per week, mostly intervals at or over LT. So rather than just ride as fast as you can, which doesn't adequately stress your system, try doing 30 second to 4 minute intervals, at barf effort. Or better, just buy the book. But yes, you are onto something. You weren't stressing yourself adequately before.