Personally, I think people can achieve much on 50-60 a week. If one can go to 70-80 consistently (my best zone) then things can get even better. I believe people can darn near hit their potential on 90-100, but you need time. Even if you could handle 90-100 you will need a lot longer time to adapt to the stress. You will see improvements, but it’s not an 8 week program and race type transition. I can get away with 70-80 and race, but not at 90-100. If I was to go to this range again I would allow no less than 8-9 months of planning.
That’s why 70-80 is my best zone. Not because it’s optimal for running, but because it’s optimal for living. I could see myself grabbing a month of this training and it being useful, but more than that leaves me flat and wore out. Why? Because my base isn’t there to support it.