I know of no evidence that fatigue, per se, plays any role whatsoever in inducing the muscular adaptations to training.
Even the role of glycogen depletion is unclear.
I don’t spend my time combing the research but I know it worked for me which is all that really bothers me in the long run 
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Yes
Yes, He said for the vast majority of untrained people 1 hr of zone 2 (LT1) 3-5 times/week was sufficient to see improvement in mitochondrial function. He also noted that an elite level athlete may need 4 hrs to see improvement. So the target time frame is flexible. More is better, but benefit may occur at lower zones. He looks through the eyes of a physiologist, so without muscle biopsy, respiratory quotient measurement, lactate monitoring, you don’t know if you are optimizing this level of training.
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Can you please provide evidence for this?
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I agree with this.
Last week I did (Seiler) Z1 rides of 3.5 hours, 5 hours 50 mins and 3.5 hours. The 3.5 hour rides feel easy. You don’t get hungry, no need to eat. As the duration stretches out you do get that fatigue and it becomes harder because of duration not intensity.
The 3.5 hour ride in Z1 will initially feel hard but you will get to the point, as you say, where you can just knock them out. Then increase the duration again and you push the endurance further out.
My minimum event duration is between 9-10 hours and stretches out to over 4,5,6, 7 day single stage events. You know the ultra endurance ones 2,000km and up.
I’m no physiology scientist. I’m not aware of anyone here who claims they are. I’ve read the stuff just like others. My experience says that the long Z1 rides are driving three key benefits for me.
- Fatigue resistance. I can go for longer and longer durations without deterioration. I stay fresh over increasing distance.
- Food intake. I’m quite happy on 20-25g of carbs per hour for many many hours. That’s a couple of short bread fingers per hour. Very manageable over long distances and durations.
- My speed at low intensity over long distances is increasing year on year.
For the multi day events this means a little snacking every hour and then more substantial meals at more traditional meal times.
The only explanation, unless anyone has another, is that the amount of energy I can generate from my fat stores is significantly uprated. I have never managed to get into this conditioning from shorter and higher intensity training.
So I’m a big fan of polarized for ultra endurance events. If someone does shorter punchier rides , maybe it is not for them.
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In your case, specificity of training and mitochondrial adaptation happily coincide. 
All of that is simply called endurance exercise training, with progressive overload. It’s only been around for at least a century.
I don’t think those things can tell you that either. Isn’t the real purpose of training to improve your performance?
Well yes, but many focus on the high intensity bit, and while you need some of that. It’s not nearly as much as they think if they are going to perform well over long distance.
The plane will not take off.
That’s a pretty niche internet joke there.
Ps I’ve gone from 5-6 hours a week to 8-11 by adding volume and extra Z2 everywhere I can get it and it’s made a big difference. Might just be me but reckon It’ll probably be the same for others- imagine that.
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What rubbish!
How many runners do you think never pass 3+ hr in one run but still have amazing endurance?
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Indeed - I race 10/25/50m TT so completely different. However, although my FTP dropped last summer as I did nothing above zone 2 until TT started, I still had some good races. I would have set a 25m PB without a wrong turn - and was still only 2s outside. Plus I set a 5 min course pb on the tough 50 I did. In addition now I’m back doing SS/over under/threshold work I find I have less cardiac drift during the session, my FTP has increased again and I feel better able to complete tough back to back sessions…why? who knows but I don’t think riding 15-20 hours week from April - July hurt
I assume those z2 additions are on the 60-90min range + extended cooldown?
Do you continually extend your long rides for progression?
I would really like to believe that (In fact I do a 70 minute ride once a week as well, but midweek and rather to maintain/not lose too much then build aerobic fitness until the next long-ish ride …)
However … maybe this analogy is completely wrong, but here goes: If I do 1 pushup a day doesn’t mean I worked on my ability to do 365 pushups in a row.
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The key point IMO is enough for who and enough for what?
For someone fresh off the couch and looking to get fitter, absolutely. For a world tour rider looking to do, well, anything other than some active recovery, no. For those of us in between those 2 poles, it depends. How adapted are you and what are you looking to achieve? Without that information, it’s an unanswerable question. The best answer so far has probably been ‘it’s always going to be better than nothing’.
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I think comparing strength and endurance are a bit different. Pushups are bit like everesting maybe without the rests on the downhill.
When you do endurance exercise you build mitochondria and increase capillairization. These changes help you in all future rides regardless of length.
For the vast majority of cyclists, a 75 minute ride is a solid ride. I have no doubt one could do a lot of 75 minute rides and then go out and easily do a 3 hour ride on the weekend.
On the other hand, if you are trying for an under 12 hour Leadville buckle or wanting to be contesting the pointy end of a race after 3 hours then you probably need more/different training. Even then, 75 minutes of Z2 is probably a decent maintenance stimulus for such a rider.