I searched for research on aerobic training of fast twitch muscles fibres and ended up reading papers from one former swimming coach and PhD Ernest Maglischo. I read two of his papers around this topic that was very interestinig:
https://swimmingcoach.org/journal/coaching-app-maglischo-vol19.pdf
This is swimming, but I see no reason this would not translate to running. Muscular endurance is muscular endurance even if running has some specialities compared.
I cannot repeat what he says, but an interesting thought is that there might be certain intensities that are very beneficial for training selected parts of the muscles (read the fast twitch type IIa and IIx, and the slow twitch, type I).
Type I might most efficiently be trained when purely doing aerobic running and as I understand this would be below marathon pace. This gives credit to easy and moderate running. So easy running has its benefits even more than recovery and junk miles might not be a case.
The opposite side is that fast twitch are trained with intensity. THe higher the better training for them, but at a cost of lactate levels that makes it not possible to do much of this training. Tabata training, 30/30 training, etc has shown great effects and it might be due to the very good aerobic conditioning of a bigger chunk of the faster muscle fibres.
Any training from LT to VO2 max and faster will increasingly employ more and more of the faster muscle fibres, but has different efficiencies of doing that. The higher intensity the more of the fast muscle fibres are employed and the aerobic adaptations of these fibres seems to be maximized. But we can only endure a limited amount of such training so how much in combo with other training is of course key and often difficult.
Any comments or thoughts on his research and opinitions?
The take for me is that I see greater benefit in doing easy and moderate running. I see the benefit of doing some easier threshold work around 1.5-2h race pace to target the slow twitch muscle fibres and spare the fast ones. I see the benefit of training at 30min-50min race pace to train the type IIa aerobically AND to make the slow type 1 to use the lactate generated (the lactate shuttling). I see less benefit in doing too much long work at VO2 max since it costs so much in recovery(due to very high lactate levels) and since the effect of 15-15/30-15/30-30/45-15 reps and 30-60s "max" with long rest seem so efficient in conditioning the fast twitch muscles aerobically. It is much easier to recover from those.
Ingebrigtsen training include a lot of this: They do easy thresholds, they do easy to moderate running, they do faster than 1h race pace "threshold" work, they do 30-40s hill work at 1500-effort and jog down and they do 200m sprints weekly (which touches into anaerobic capacity for fast twitch)
Maybe this is not revolutionary, but just that explanations for efficient training is catching up the good practices.