Every kind of training can run argument with pros. You might elaborate hundred reasons why it works. But the long run side-by-side with other training formats, the long run can´t win against.
After read your detailed argument pro long run no doubt that you insist on the long run defense it´s simply because you do the Lydiard stuff.
Obvious that every kind of activity, might contribute to distance run performance improve. Every kind with no exception. However some kind of activity is more adequate and with an high degree of connect than other.
THIS MEANS THAT YOU GET BETTER AT WHATEVER YOU DO IN TRAINING. The plyos, the long run, weight lift, walk, every kind of training contributes to distance run performance improve. However because that activity are distant to the pace and movement pattern of the run event that is weired and not specific relate to the run events.
It does seem to be pretty certain is that the RESPONSE TO TRAINING IS TRULY SPECIFIC in terms of movement pattern and energy system utilisation.
The long run training middle distance runners goes to that category. It´s one activity that isn´t specific relate to every distance event from 800m to HM, because it´s “slow” relate to the pace of that 800m to HM runs. If you run for 2 hours or 22 miles long, and despite that you don´t “jog” you run either, and eventually that last period it´s all-flat out, what´s the best/fast pace can be sustained ? It´s slow for sure, relate to the pace that the same individual runs 800m to HM events. Consequently the classic long run isn´t considered specific training or with an high-percent of specificity.
Peter Snell says about the long run: “the benefits of distance running are achieved after muscle glycogen depletion. So if you run for two hours a lot of the slow-twitch muscle fibers which were initially recruited run out of glycogen and cannot contract any more. Eventually you use the fast twitch muscle fibers which you normally only use when running faster.
With one short run you can get the more benefits if you run faster and longer but not as length as the long run, AND you run long intervals or long fartlek. Going fast-easy-fast-easy you got depletion, the ST fibers and the FT fibers are recruited as well, however the pace is faster during the fast bouts what means more specific and FT recruitment and during the periods you go easy you recruit the ST fibers.
Let´s say that instead of 2 hours run, you go for 1:30 hour but the same runner that able 6:30/mile pace for 2 hours, runs 1:20 but one IN-OUT fartlek of 5min/fast-5min/easy.
What are we looking for ? We are looking one stimulus, one kind of activity that gives endurance, stamina, while can get more benefit and be more specific. Ther´s no doubt that some run workouts that are different than the 2 hours long run are more benefic than the long run, because they have the principle of specificity. The closer is the pace relate to race pace, the longer you run on that pace it´s specific on the energy system.