Salazar's double linear periodisation, Canova's special period —> specific period, ... though they use different means, they all focus on training specificity. The training is specific to the event the athlete is competing in. Early season training is designed to provide the figurative 'base' either side of the goal pace (previously the weekly 20min tempo which is now being pushed up to 8-10 mile of tempo reps above 1/2 pace, and short sprints on the track to maintain speed) to allow for the heavily emphasised block of specific training. We all know what these session are, every high school, college all over the country does them: 5x1k at '5k pace' (athletes often pushing up to far faster than their realistic 5k pace), 5x600 at '1500 pace', 3x400 at '800 pace' often huge rest and an accompanying focus on VO2max work – often hands-on-knees, soul-destroying work. This is what most of the top pro groups (such as BTC, main Ethiopian group) do now (but generally a far more nuanced, often more controlled version).
But Jakob has come out and crushed them (runners who are far more elastic, employ better tactics and, frankly, more talented, than him).
Tuesday AM 5x2km at threshold.
Tuesday PM 10x1k at threshold.
Thursday AM 5x2k at threshold.
Thursday PM 20x400 at threshold.
Saturday AM 2 x 10x200 hills.
Count that up. 38 KILOMETRES of threshold work (+4km of hills). All under 3 minutes per kilometre. No specificity at all, in fact, no hard continuous work longer than 6 minutes. No work in the balls-to-the-walls 'VO2 max' 3k-10k pace. And of just this 'base training', Jakob runs 13 flat at sound running. And as for specific training, all Jakob does is 10x200, 10x300, 10x400, etc... at 1500 pace, just more 'relaxed' pace work, allowing him to feel relaxed at pace but not destroy him.
So what does this show? Training is not about specificity, it is about pushing up the limiting factors to running faster: namely decreasing amount of metabolic byproducts present (how to do this? push up threshold), improving ability to tolerate metabolic byproducts, biomechanical comfortability at pace etc..
And how do we achieve this? Tailor our training towards what we are trying to improve, not to practice more and more the race itself. Is the Norwegian model the only way to do this? Certainly not, but nobody has put together 40km of threshold work most weeks a year for 5+ years, controlling the training enough not to overtrain, and keeping a hold of ability to run at pace.