We continue to learn and know a lot more than we did 10 years ago. We know that High Intensity Interval Training is really a key. Your body develops in response to stress which is why High Intensity Interval Training works so well. While it's true that a cyclist needs to break his butt in and you need seat time for that, in terms of racing and raising your anaerobic threshold and basic cardio improvement, it's all about stressing your system by pushing yourself to your limit. I would recommend "The First Twenty Minutes" as a great read that helps to explain how important it is to do high intensity work, as opposed to long slow distance work. Both have their place to be sure but it's the stress of HIIT work that really transforms a person. Cyclists typically get that when climbing hills even if they don't consciously work at it. As the book said, if you ride easy for twenty miles 5 times a week you'll get comfortable with it but it won't continue to get you better and better. Stressing your body for 30min several times a week can take less time but you'll continue to improve by doing that to a greater degree than when doing long slow rides.