DK, the following comes from not a whole lot of experience, but time reading this stuff recently - your own ramblings, MYI's, any thread mentioning Friel etc.
Firstly, some of the riders i ride with have extraordinarily fast times to very near peak fitness - ie for me, while its months of work, i ride with a guy who can step up 25% in 4 weeks - starts 6 weeks out (has long history of triathlons, etc, never carries excess weight either).
Next, the guts of a lot of stuff seems to be you get the best value training at around your LT or FTP, either 10min, 20min, or longer sessions (shorter ones obviously have rest intervals between). You train at these points because your max output in short intervals will improve towards your genetic maximum with this training, so there's no real benefit in doing really short max intervals - except in that it build a bit of muscle, but its not part of your cardio system that you can improve (ie i am slow over 2 minutes- i always will be, and have always been)
Overtraining - the guys i have read that depend on scientific study rather than what some coach said, seem to have a one day hard, one day easy kind of regime (this also came from a friend who is currently representing australia in xc running in europe). The Friels approach to the same problem is to monitor your resting heart rate in the morning - know what it is over a week, and monitor it - after a hard day (maybe a 40min FTP ergo session) it should be higher than normal by 8 to 10 beats - this is the signal that today is recovery/easy day - that is around 85% of FTP/LT or 75% of MY MHR (keep in mind that my LT is 89% of MHR - yours could be higher or lower. Conversely, if your RHR isnt out of normal, go ahead and train hard for the 45min or so where you get max benefit (law of diminishing returns seems to set in quite heavily at 45min).
So if you are looking for a level to train at to maximise your return over short training periods over a short length of time - do a test and figure out your LT FTP. If you just want to avoid overtraining, do your resting heart rate and track it - this will also be a score to track your improvments (1.5yr ago, mine was 68, now 50) so MHR - RHR = performance gap available for cardio system - bigger the number the fitter you are.
hope this isnt all too basic for you, but it summarises a whole lot of what i've read over the last little while