Progression is extremely fast. I told before on this forum, that I had 3 years cycle training as a amateur. Max power what I got is 320 FTP. But I did a lot of base building (zone 1,2). Total hours on cycling is about 1000 hours. During base building on cycling I did sometimes 30-35 hours per week in zone 1,2 only. And of course, I had lucky to have in my place good mountains. Without mountains it is hard to build muscles.
Same in running, I understood, gradient less than 1% all of your running - is nothing, you need more. Running on track with 0% is not fully effective for your fast development, need hills, constantly.
Artur Lidyard no have such big success when he moved from New Zeeland (hilly terrain) to Europe (flat) and coached there. Hills is the one of the important key.
By myself, I noticed this when I moved from Tenerife (1%+) to Valencia (0.3% avg gradient), performance dropped, and than I back to my home in Russia, where again all my runs were at 1.0% and more gradient, I noticed again increasing performance even at base building without workouts. Same I noticed now, when I am training on treadmill, I use incline 1%, but as you know vessel has a trim... and when I checked by cellphone inclinometer how much real incline, I was shocked - it was 2.2-3.0%, sometimes 3.5% - and so huge and fast progress as a results. So, all this 4 months during running on treadmill I did not know, but I was trained at a huge gradient at all runs (easy+workouts).
Another thing: treadmill is super effective pacemaker. Ok, if you have a good partner on land which is running faster with you, he can be your ideal pacemaker, without pacemaker also is hard to keep good progression. I am starting to think about one good idea from our forum, idea to use drone as a pacemaker! Or otherwise during workouts on land you need to use treadmill as well, as a pacemaker (fast treadmill up to 25 km/h).