Hopefully Dr. Daniels will weigh in here eventually. But so far, this has been a great thread.
The Rep workouts can certainly be demanding and this has been a concern of mine.
My interpretation of the DRF is that Phase II's primary focus on Reps is intended to improve an athlete's economy in preparation for the Interval (VO2 Max) focus in Phase III while minimize overall stress to the athlete.
My concern is the length of the perscribed Reps. There is a significant difference between 200 Reps (between 30-40 seconds) and 400-800 (between 65-2:30 seconds). Lactic Acid appears to accumulate and "take it's toll" after about 35-40 seconds. Once this threshold is crossed the body must shift focus and find a way to deal with the LA build up, rather than maintain economy. This is clearly evident in the end of most 400m races. This shift at about 40 seconds is often to the detriment of proper form and thus economy.
It would seem that Reps of say 30-40 seconds (regardless of distance) would enable an athlete to develop economy without crossing into the "No Man's Land" of excessive LA buildup.
There is a clear message in DRF to train with an express purpose and therefore within the scientific parameters of intensity and duration. Wouldn't the purpose of Phase II, to improve economy and prepare for Phase III while limiting the overall stress to the athlete, be jepordized by long (45-2:30) Rep training?
This may explain why a number of HS coaches using DRF have noted there athletes often peak too early in a competive season. As Lydiard noted, the body can only managed a higher ph for a limited period of time. Perhaps if Phase II were modified to avoid LA buildup by using only Reps of 30-40 second duration the Phase II purpose of improving economy without excessive stress could be achieved more effectively.
Now if Phase II were completed in this way (avoiding LA buildup), the similarities between Lydiard and Daniels would become more apparent. Phase II of DRF would be appear much more like the transitional Hill Springing Phase of Lydiard, with each phase preparing for Anaerobic or DRF "Interval" training.