Fair point, and I didn’t mean to sound like a know-all saying base miles are needed as I’m certainly not, just going off a lot of advice saying it’s a good thing and my own experience. Perhaps for running there’s less potential for work in z2, it’s easier to regulate effort on a bike? I don’t know. Learning from threads like this.
I looked for some info about the changes in cardio system that base miles are said to build (an increase in the capillaries network that is then worked on in HI training, HI just works what you have? I may have misunderstood that, gets a bit scientific), so if it also happens doing sweetspot level work then base miles may be less important for many racers, but couldn’t find anything so far. That’s the main reason I stuck to base miles last couple of years and tbh the results later in the year were better than when I used to do half as many hours and tended to ride faster more of the time – probably around that sweetspot area of mid z4.
Another thought was that the z2 work may not be needed for a basically fit cyclist as those cardio changes only happen to an unfit person starting out? I’m mainly concerned that less z2 long rides and less time but higher intensity will not be good for endurance, feels like it’s taken a long time to build that up from where I was. And that’s a different aim to yours I think.