Devoted Daniels acolyte here, but a few notes on the third edition.
First, let me issue this disclaimer: I haven't read it. But I have read enough on these boards to pick up on a few things that bother me about the 3rd edition.
First, I understand that he goes away from straight 20 minute tempo runs exclusively (or almost exclusively) in favor of cruise intervals. I assume he does this to reduce the physical and mental stress associated with tempo runs, but the mental stress is one of the things he emphasized in previous editions as a benefit, and I agree with him there. Few things can prepare you for the pain that comes at the end of a 5000m race, and the discomfort that comes at the end of a 20 minute tempo run is one of those few things. It really forces you to focus on pace and effort while fatigued. I cannot overstate how invaluable that has been to me over time.
Second, he dropped the table from the 2nd edition that helped figure out how much to slow down to go from 20 minute tempo runs to longer runs. Presumably he did this in connection with the previous point. While this in and of itself is not a big deal for 5k/10k training, I think that 30+ minute tempo runs are great for half marathon training for many of the reasons that I mentioned above.
Third, he has added in marathon pace running a lot more for non-marathon distances. This confuses me since it falls squarely in the "junk miles" category described in previous editions. The only way I see that being useful based on his previous editions is if it is being used for an extended period, essentially replacing the long tempo runs described above.
Finally, this idea of back-to-back workouts. I do think there is value there for folks who have to run heats before a finals or who might double in a meet. And while it is not in previous editions of his training plans, I know my college coach (also a Daniels devotee) occasionally had us do this, so it may not be a brand new concept for him, but it does seem to be contrary to his position on recovery.
I honestly have to wonder if these changes are the result of new research, or if they are being done to accommodate current trends in training and keep Daniels relevant and to justify a new edition, all in an effort to keep selling a book. Based on Daniels' age, I suspect that it is more likely that he is just staying relevant than he is doing new research that is resulting in modifications to his prior approaches.
I feel dirty for blaspheming with respect to Daniels, but I had to get that off of my chest.