I follow a plan built using the principles contained in the book fast after fifty by Joe Friel.
He covers three basic intensities of intervals; aerobic capacity, lactate threshold, and aerobic threshold. He covers nutrition and recovery. He covers testing. He covers periodisation. He covers efficiency factors. He covers strength work He covers a few other things as well, including how to build a plan
It contains some great advice, particularly if you miss an interval or cannot complete it. He also covers what the priority workouts should be during each period of training. The ones you should try and do, even if you miss lower priority ones.
Till this year I had never done structured training, but I am really enjoying it, and enjoying the results I am seeing.
Joe Friel also has a new version of the Cyclist's Training Bible out. But so far I have found fast after fifty and building plans using knowledge contained within works well for me without getting too in depth. Though he does cover the biggest causes of decline in performance after fifty.
I listen to music during the workouts, but recently tried podcasts. Sherlock Holmes does not work, too complicated a plot in the middle of an interval. Adventures series, Kayaking the length of the Columbia River worked very well for me.
I am surprised as how well I have taken to it, but I think this is in part, because it allows me to enjoy my audaxes and road rides without worrying about how hard I am working. I can just enjoy the mental relaxation and scenery and other aspects of being on the road with no other concerns.