Originally Posted by AlgarveCycling

This is certainly true for stamina and endurance, albeit you absolutely need to bust a gut and do HIIT for speed increases for racing, TT's etc - you cannot expect to increase outright speed purely by riding Z2 albeit you will go faster with increased stamina over a longer course - but we need both to go really fast. So if it is simply a matter of completing 100mi then volume at a steady pace is just fine. If it is to increase racing speed, then HIIT needs to take place.

Pogacar's coach explained this recently and GCN posted a video talking about it

There are a couple of new-to-racing chaps I know, both in their early 30's, both very strong athletes, who have decided full-gas is the only way to go. One is a sponsored rider, plenty of natural talent, the other used my coach for a month and then decided he didn't like being told to do slower, easier efforts - Z2 was not for him. He is laser-focussed upon impressing with high average speeds. He likes to suffer and has an enviable ability to do so. However, I predict both these guys will burn out and stop improving in the not too distant future. I know, I've been there myself when I was younger. Overtrained.

Z2 is very important and I do more of it than HIIT but both are required if speed and endurance are the goal.