Wow, there is a lot of great, thoughtful, physiological discussion going on here!
I like the questions and thought process here, but I wonder if this might be thinking a bit too much into it? Taking a step back and thinking about the SUF approach to training, it’s basically polarized, where we program the most amount of training time in zone 2/aerobic/general endurance, and depending on the plan, what’s left is either high intensity intervals or tempo/threshold intervals to increase fitness and power at FTP, MAP, AC and NM. So if we’re actually adhering to our training plans and spending a lot of time in zone 2, which is our “all day” pace- my first thought is that this is not going to be the limiting factor for 99% of us when it comes to race day. I mean, if you can ride “all day” at this pace/effort, then how are intervals at this effort going to stress the body enough to produce an adaptation? I could see this being a good way for new/beginning cyclists or riders who’ve taken an extended time off the bike to increase their aerobic fitness as they get into the sport and structured training, but for riders who have been training for several years and already have enough aerobic fitness to ride 50-100 miles, I’m not convinced that intervals in zone 2 will be terribly effective, especially considering the amount of time these workouts take and the amount of training time most people have in a week. When it comes down to it, I think raising FTP and MAP will have more of a benefit for most people as those are the greater limiting factors when it comes to racing and performance.
*Remember this is coming from my coach brain, where I’m thinking about the most effective strategies for most people, rather than the small percentage of people who might benefit from implementing a technique that may or may not make a difference…If you have plenty of training time on your hands and want to give it a go, then I’d love to hear your thoughts and results. In order to do so, I’d make a plan using our building blocks in a linear periodization approach. Start with 2 base blocks- executing the first one at the intensities as prescribed, then in the second block add intervals of 5, 10, 15, 20 minutes at that high aerobic/low tempo zone of 75-80%of max HR, then moving on to the tempo, FTP and MAP blocks.
But also keep in mind that changing up your approach to training is usually enough in and of itself to stimulate some improvement in fitness. So whether you try this method or not, remember to take whatever results you get from it with a grain of salt!