I do agree with the bucketload of low intensity riding though
Then we don't disagree. The question was about Z3 training, "no man's land". From that study I previously cited:
The available evidence suggests that combining large volumes of low-intensity training with careful use of high-intensity interval training throughout the annual training cycle is the best-practice mode
The biggest danger is this:
Comparing the intended and achieved distributions highlights a typical training error committed by recreational athletes. We can call it falling into a training intensity “black hole.” It is hard to keep recreational people training 45-60 min a day 3-5 days a week from accumulating a lot of training time at their lactate threshold. Training intended to be longer and slower becomes too fast and shorter in duration, and interval training fails to reach the desired intensity. The result is that most training sessions end up being performed at the same threshold intensity. Foster et al. (2001b) also found that athletes tend to run harder on easy days and easier on hard days, compared to coaches' training plans.
That "same threshold intensity" is z3, which *feels* like hard work, but it's that no man's land.
Freddie's point is that before you even get into the z4 stuff, you should have a well-developed base engine -- something that few recreational cyclists have. He himself lives that -- genetics may have gifted him a huge engine, but making sure it is properly developed is his top priority. Race-induced intensity will get him that last little bit of performance. But when he trains, it's all z1-2.
Once you have no cardiac drift, then intervals and other high-intensity work will squeeze out the last few percentages of your genetic potential performance. There should be nothing controversial about that -- it's the whole idea behind base training, and the myriad "ride slower to go faster" headlines in the cycling press. Thing is, most people don't do that. Z2 is boring.
As to why I would get injured with more aggressive high intensity work, like I said, I'm not a naturally gifted athlete. I have fragile knees and lower back issues. High-Intensity work aggravated those problems. I also got sick more often. And my motivation would suffer. None of that is an issue with my low-intensity program, which means I get to spend more time on the bike training instead of recovering, hence consistent improvements. I just finished Strava's 500 km challenge for the last week of the year. In my pre z2-training life, there's no way I could've gone out and done a week of (mostly) consecutive 60-70-mile rides.
Also important for me: getting good lights so I could train at night, after my family goes to bed, was huge in getting me the training volume I need for this sort of plan (10-14 hours a week). I'm not a morning person, so I wasn't about to get up early to train.