Not a criticism of all academics and I love dr. stephen seiler but his advice isn't generally actionable for most riders, especially those time crunched trying to race on 4-5 hours per week. He waxes philosophical and always has. What I learned from his papers perhaps 8-9 years ago was to do at least 90% of my time in zone 2 or lower and less than 10% of time at high intensity. In terms of percentage of sessions, 80% at least in zone 2. So, 4 long endurance rides for every short in time HIIT session. I bought a lactate meter to measure the LT1 transition. What was really shocking is how much LT1 increased just simplifying my training. Seiler's points on ANS balance are something I've been thinking about and thinking of not doing my intervals (5 minute VO2 max) as hard but that just runs counterintuitive to me. 3 zones runs deep in my brain. 2 Zones? ??
ISM on the other hand gives more specific advice albeit along the same vein. Both say rest is rest and to rest. ISM says he tries to ride 5-6 days per week. Only goes like hell once. 7-10 hours per week like that is probably enough for a sportif rider. Clearly not specific enough and not enough volume for someone trying to be competitive in racing.
Couzens is the one with tons of real data. Progress should be measured in years and consistency is the most important training attribute in his opinion. Year after year. His data also says volume in zone 2 is key to all improvement.
Famous older marathoner Ed Whitlock did a ton of volume as slow speeds. His VO2 max and marathon times might eventually be broken. One thing though is he raced shorter races closer to record attempt marathons (5 and 10K) and this could be seen as the specialization or the pyramid in what looks like all zone 1 and zone 2 work when seen from a macro lense but over the few months leading to phenomenal races, it was very specific. IIRC, he ran a sub 3 hour at 95% of hi 52 VO2 max into his 70's or something along those lines. Astounding.