I have read books by all three, and they all say to run threshold runs of 20:00 at a pace that's 25-30 seconds slower than 5k race pace, or about the pace you can run for 50-60min all out (which coincidentally comes out to be about 25-30 seconds slower than 5k pace). I don't think they really know what they're talking about because my coach always has us run a lot faster than that for our 20:00 threshold runs. He usually wants us to run only 16-20 seconds slower per mile than 5k pace. I also remember watching Fam's "Run Like Hell" and how Fam's highschool coach had him run workouts at a level 4 above where Daniels recommended, and it seemed to work for him.
Who's right, three exercise physiologists or the REAL coaches?