Yours is one of the most common questions we are asked. You are right that when we wrote the book we were trying to demonstrate a “best practices” sort of strategy. We wanted to explain what we had used for ourselves and why it worked and how it might be scaled to some extent and still stick to the underlying theories. Since then we have worked with many, many other athletes coming at their training with more time constraints and less basic fitness than we were experienced with. We’ve learned a lot in the past 4 years and hundreds of athletes.
Some of the stuff we’ve come to understand better now:
1) Our fundamental principles work across a huge range of athletic backgrounds and ages. We have enough empirical data from working with ordinary working folks to say with 99% certainty that you will see gains if you follow our programs.
2) The minimum time to make substantial aerobic gains is not possible to predict accurately because too many factors will affect that adaptation; mainly genetics and training history. But I can say that we typically see a significant increase in aerobic capacity within 3-4 months for people doing 6-8 hours of training under their aerobic threshold (top of Z2). These gains come in fits and starts but what we normally hear from clients is that one day they suddenly could tell that they were running 10-20 sec/mile faster that their typical pace. This process can be sped up with fasted training. I had one athlete drop his running pace at AeT from 8:30/mile to 6:15/mile in 4 months of nothing but aerobic base training using Z2 as the upper limit. His AeT HR went from 135 to 165. He is a high responder to be sure but 30sec/mile improvement is not uncommon to see in 3-4 months.
3) Duration is the biggest stimulus for this sort of aerobic base training. So 10 hours is good but yes 20 will be better if you can manage the physical stress (let alone the time commitment) We have professional alpinists who train with us doing 25 hours in their final weeks before their main goal climb.
3) Don’t make the mistake of thinking that you can accomplish this same end result with high intensity training. HIT is working a completely different system and while it can be very important for many athletic pursuits it is far more effective when applied on top of a big aerobic base and not when used as a substitute for one. Moderate amounts of high intensity training is not only justified but desirable for increasing fitness ONCE this base is built.
4) No endurance athlete can just keep on increasing volume forever. It provides diminishing returns and there is not enough daylight. At some point all have to add some intensity, change the stimulus. What was once a good training stimulus will, once adapted to, no longer provide that same stimulus and if the training stimulus stagnates, first the training effect stagnates then will drop.
If you can hit 10 hours of aerobic work in a week near then end of you 20 week plan then you should definitely seek good gains. You see even more the more times you run through this training cycle. The next year you’ll start in a higher place and move even more up.