Arthur Lydiard was fixated on the magic number of 100 miles a week, and the hills wound these 100 miles from despair, in view of the lack of flat terrain in New Zealand. Moreover, he criticized Percy Cerutti (Herbert Elliott's trainer) for using mountain running as a means of training.
Applying Arthur Lydiard's training methods in their purest form outside of New Zealand did not have the effect he had hoped for. Neither in Mexico, nor in Venezuela, nor in Denmark, where Arthur Lydiard worked as a coach, he did not achieve success. And in Finland, the results appeared after his departure, after Percy Cerutti's training methods were added to these methods.
Hills (gradient) is the one of the important key of Lydiard's training method.
From my own little experience, I always had less progress with cardiovascular system and muscle system when training in Spain (flat terrain=0.1-0.4% avg gradient of all my runs) then at my home in Crimea (0.8%-1.2% avg gradient of all my runs) or treadmill (1.8-2.5% gradient).
Same I can tell you when you are training in cycling sport: Mountains are an integral part of any road cyclist's training and the ratio of gradients is different than in running, the gradients are much larger.
If you have your terrain avg gradients are 1.0% and more, you are very lucky ???