Runningart2004, you are so right about finding a training combination that works for you. If it were in a textbook, everyone would get faster and no one would ever overtrain. The mentally toughest would always end up being the fastest by working the hardest.
Unfortunately, as we all know as runners, this sport doesn't always work that way. Every type of workout has pros and cons.
So, as a (hopefully) final post to this thread, let me give some opinions:
Tempo runs: If done as a single 20 minute or so run, gives a single sustained stimulus to the anaerobic energy system and requires your body to buffer a significant amount of blood acidity. Physically, it's probably the easiest way to stimulate the anaerobic system. However, for the reasons I have stated in earlier posts, I believe can lead to overtraining in a very deceptive way.
Cruise intervals: Break the tempo run into 3 x 6:30 with some rest in between and you have given your body three stimulations and (hopefully) allowed your body to reduce blood acidity between stimulations.
Fartlek / Distance runs with surges: IMHO, the best workout there is, for a lot of reasons. I really like the idea that I can hammer hard for about two minutes (anaerobic), then let things recover fully before thinking about doing it again. Or, I can throw in a moderate surge where my breathing increases (aerobic), and then slow back down as soon as I feel weakness beginning.
I'm out now, because I'm not trying to prove that I'm right, but I hope this helps others find some answers for themselves.