I think the whole 80/20 concept is not really well investigated and published and maybe even dangerous to follow. The study's always use three zones to classify training intensities. Zone 1 under 2 mmol, zone 2 between 2-4 mmol, zone 3 over 4 mmol. You should now do 80% under 2 mmol, and the rest over 4 mmol. So you do easy runs and intervals at 10-3 k pace. But what exactly is easy training intensity.
Take for example Stefano Baldini. 2 mmol was at 3:00 min/km. So what exactly is easy for Baldini ? Should he run 4:30 min /km- 4:00- 3:30 min/Km or 3:20 min/km. According to the studies it doesn't really matter. All aerobic running for him. Baldini had 4 mmol at around 2:45 min/km. So 20% running at a pace of 2:40 min/km ? You see my point. The whole concept oversimplifies training especially for elite runners. They have such huge speed ranges of easy paces.
Then theres Kawauchi, daily running at 5:00 min/km+ intervals at 3:00 min/km and races. Another perfect example for polarized training, isn't he? A lot of running under 2 mmol, some stuff at 2-6 mmol (incl. races).
The same als Baldini, a lot of training under 2 mmol+ some training at 2-6 mmol. this is the perfect prove for polarized training.
Yes, if we just have 3 intensity zones. But there training paces are totally different. Baldini running normally around 3:30 min/km for daily runs.
I think a big problem is calling zone 2 medium intensity. For an elite runner and even amateur runner medium intensity is still far under 2 mmol lactate and therefore slower than marathon pace. But in all these studies they have medium intensity at marathon/half marathon pace. Sorry, but this is not medium intensity for a good runner.
The terminology works for joggers. They are running marathon speed for there daily runs because they are so unfit.
I think the should study the effects of different easy runs intensities. Is it better to run at 65-70-75-80% of heart rate, should you mix it up,
is traditional base training usefull, does speed work kill base etc. There's so much pseudosciene in exercise physiology it's disgusting.