@anthonylane That is true, and of course you get MORE benefits the more you do. That’s not to say that shorter sessions at Z2 aren’t useful. I completely disagree that the polarised model is a waste of time. It is for SOME athletes, but for others it’s necessary. It is completely dependent on the individual’s profile and their goals.
Coming back to the original question, what we’re after is volume over time. We’re not looking for one-off massive trainign days or weeks, but consistently high (appropriately) workloads over years. A 45 minute Z2-workout done on any given day may not be worth squat, but if you do it twice a week every week of the year that’s 78 more training hours in the year. That DOES make a big difference.
Also, although the downstream effects of low- and high-intensity exercise are more or less the same (since both end up triggering the same master signalling switch), they do so in different manners (high-intensity playing the AMP-switch and low intensity the Calcium-switch), so neglecting to do the endurance work may lead to you leaving a lot of untapped potential on the table.
1 h @ Z2 is absolutely enough to trigger that switch, which also means that it becomes a stimulus for improved fat oxidation.
100% agree with what you’re saying about TSS though! But that’s what I said, or tried to say in my initial reply as well. The more I learn the more I start to think that TSS is massively oversubscribed and downright misleading oftentimes.