I feel like what this thread is really uncovering is getting to the bottom of what training really is. We should all be individualizing more. There are certain shortcuts or benchmarks people use as a guide for how to train which are really quite false and bad but are very common:
1.) Using a race performance to modify training. It should be done in certain ways but not in other ways. Where this goes wrong is when the race performance is good and people use this as a reason to modify their training. Isn't that just crazy? Good races are often used as justification to increase training intensity or volume, like "I need to update my training paces according to this new performance" or "I have achieved all the improvement I can get out of this training volume and now I need to increase it if I want to continue to improve." Both ideas are insane! But they are so common. If you have a good race, that is an indication that your training is effective as it is! Don't change it! It's only when you have a bad race that you might say "I'm overtraining" or if you have plateaued then you might say "I'm undertraining." If you have a good race, your training is effective, don't significantly change it! Run the same volume at the same perceived effort. Paces and mileage will naturally increase without conscious effort. Keep doing that until improvements stop coming, and only then do you intervene.
2.) Comparing your training to someone else's. This is also crazy, especially when you tie it in to comparing your races and your training to someone else's races and training. Our ability to train is way too individualized, just like our ability to race. One untrained person can run a 17:00 5k while another runs a 23:00 5k. One untrained person can do training load X while another untrained person can do training load 2*X. It can really vary by that much such that it's not even close. And the two things (our natural ability to race and our natural ability to train) aren't related. The 17:00 5k runner might be that much faster in the 5k but they cannot handle anywhere near as big a training load as a slower 5k runner. In fact I'd guess that they're probably inversely related: the faster you can run with less training, the lighter your training should be and the more cautious you should be about increasing it.
3a.) Key workouts. People do "indicator workouts" or "race simulator workouts" which are totally unnecessary and completely break the optimization of training (in the sense that the amount of time they take to recover from is not worth the benefit they're providing). RACES are the thing which we try our hardest in and break the optimization of training to do. If you want to do hard efforts, then do a race! Don't race in training! People need to have confidence that their training is effective. Jakob has talked about this -- he doesn't need to prove anything to himself in a workout. He's 100% sure that the training he's doing is effective. His hardest efforts are reserved for races, where they matter. All the rest of his effort is used for optimal training, which doesn't involve maximum efforts.
3b.) Peak weeks and "down weeks". This is so high risk and is there really a reward? You're essentially overtraining on purpose. What if you don't fully recover? Your race performance will be ruined. What if you would've gained more fitness by just having two normal weeks? Marathoners will increase and increase their mileage and then have a huge marathon pace long run "on tired legs" for the peak of their training. Maybe, MAYBE, designing your training cycle with this kind of thing is optimal. But SO many people mess it up and it is absolutely not necessary. 99.9% of runners do not need to mess with this concept at all. Not every training week should be identical in nature (there's some training that's better done far away from your race and some that's better done closer to your race) but no one should be overtraining on purpose, even if they're scheduling extra recovery.
My main takeaway from this thread is that having 3 quality days a week and running really truly easy on the other 4 days is a very effective foolproof template that everyone should try to follow. And in order to follow it, the quality days have to be easy enough to recover from that you actually recover and can sustain the training. And in order to do that, the easiest type of workout that you can do which is still effective is these easy threshold intervals, starting with as small a volume as you need to sustain it and building up to what Jakob does as a maximum volume. These sessions are what you can do on the days which precede 1 easy day. What you do for the 3rd quality day of the week, which precedes 2 easy days, can be a little more difficult and should cover a different aspect of physiological development (like doing hills / speed, or for a marathoner, a long run with some kind of tempo / fartlek).
So as someone else mentioned, trial-and-error is very likely the best thing an individual can do. Doing some kind of fitness test, max heart rate test, monitoring heart rate or monitoring lactate, etc etc, should just give someone a starting point. And then it should be individual trial-and-error: can I do my 3 sessions a week indefinitely? Am I really staying on top of my recovery? That should be the top priority. Make it easy enough so that's a confident "yes".
After that, you occasionally race. If your performances are improving, then change nothing (except what naturally changes by keeping the same perceived effort). If they plateau, then think about how you want to increase the stimulus so you can continue to improve: maybe your paces have slacked off a bit and should increase, or maybe you should increase volume a bit. Either way, make a small incremental change, make sure the training is still indefinitely sustainable, and race again in a few months.
There's NO NEED to continuously compare your journey to anyone else's or to refer to a chart or any physiological measurements or anything. I understand that doing so has been massively helpful for Jakob and Gjert says it's essential, but they're attempting to reach absolutely 100% of Jakob's potential, setting literal world records. And copying them might not even work for our own physiologies. They've made no attempt to provide a universal blueprint that will work for any physiology. We should guide our own journeys. Start somewhere -- err on the side of the training being too easy -- and see if you improve or not. Get an idea for what you can handle and increase it only if necessary.
Anyway, that's my takeaway. I personally have been the guy who runs relatively fast on little training, who has then taken cues from other peoples training (and not just on my own in some misguided and incorrect fashion -- I've received individualized training from coaches who have coached elites, and from elites who have gotten into coaching) in an attempt to get more serious and perhaps graduate from "sub-elite" to "elite", and then just ended up overtraining over and over and over again. If that rings a bell, I think this is a good lesson to learn. If that's not you, maybe my perspective isn't helpful. I'm too old now to ever be elite (well, there's always the masters category) but finding good training late in life is still a great gift, better late than never.
I respect the people who are keen on comparing training and races in order to build general knowledge and understanding. It's an essential part of all of this. It's a worthy project and could eventually lead to comparisons actually being more useful than they are now. But for an individual just managing their own training, my strong opinion as someone who has been led astray so many times, is to use it as a starting point and then live in your own individual world.
PS: I have a pet theory that a lot of sub-elites and low tier elites have been overtraining their whole lives, but not so bad that they don't improve, so they just keep doing it their whole careers. Then at some point they stop improving and they cannot possibly train any harder, so they think "Well I hit my genetic peak. This is it." But they're wrong -- they could've hit a higher peak by training lighter along the way. Some of us can't improve while overtraining but I think a lot of people can and then they develop a totally warped idea about the relationship between training and performance, and that trickles down and permeates to everyone else, creating all kinds of false ideas about what kind of training is necessary to achieve a certain performance. I base this on the fact that (like I said earlier) people keep increasing their training loads before they plateau. They have so much fear about "wasting" a season of training by undertraining that they constantly err on the side of overtraining, but not so much that it's catastrophic, and then they end up spending their whole careers slightly overtraining, never actually training optimally. They burn out and/or plateau before they accumulate as many years of effective training (before getting too old and declining) as they could have.