Great questions! TL;DR Any training in Z1 or Z2 will help you move the delta between AeT and AnT closer to 15%.
As you said AeT is less a number and more a range. As someone else said in the forum, Zone 2 a proxy, and I am paraphrasing here, for the the theoretical high end of a metabolic function, of processing glucose and fatty acids into ATP at the cellular level without pushing lactate into the bloodstream. However, it is important to remember that Z2 training is the _stimulus_, but there are many adaptations your body makes to that stimulus during recovery. The adaptations your body makes are due to acute and chronic stimuli. In my opinion, the most important adaptation we are making in this phase are connective tissue strengthening built through this kind of low intensity training which will allow us to train safely when we start getting real loads on our back and going uphill. OK, enough exercise phys…
“What’s the optimal target for sub-AeT training?”
You have it right, it is a range, and err to the the lower end. For instance, my AeT is 126, I use 125 as my AeT but anywhere between 110 and 125, has worked for me. If you want more details on why I think going erring to the low end is better than erring high, let me know.
“Does it matter much where in Z2 we accumulate base hours?”
In my opinion, no, it doesn’t matter. More accurately, I should say that I haven’t noticed that in my training. Especially, if as you say, your AeT and AnT are far apart, anywhere in Z2 is perfect. Also remember that I think Marc said in the call that 10% is optimal, and that 15% is good.
“By extension, how much does Z1 training contribute to building one’s aerobic base?”
I don’t know about aerobic base, but it is non-zero, I’m guessing, as part of a chronic stimulus. However, Z1 training is a very valuable part of my training. After I go on a shorter walk, 20- 30 minutes, my hips, back, really everything, feel much better and I feel better the next day so I am ready to go for a harder workout if that is what I am doing.