Originally Posted by Steamer

My emphasis above. To that statement - yes and no. I think it depends entirely on the type of event one is trying to perform in. For me, a training diet of 95% z2, 6 to 9 hours per week, from December to May, had me performing better in spring brevets than I ever did before. In past years, my preparation included a lot of z3, z4, and z5 riding, but my endurance and 'durability' was poor with that older approach. I was putting in slightly less time on the bike overall then. About 5 to 7 hours per week. GhostRider62 was very helpful in providing inspiration and guidance to try things a different way, and I couldn't be happier with how it turned out. My z4 and z5 performance probably sucks, but it's basically irrelevant, for me, as I would not typically alter my chosen pace at any given time to try to ride with others, be they slower or faster. I did a lot of solo riding in those brevets, but that's fine by me. Comparing my riding between last year and this year, I am now riding a bit faster, with a 10-20 bpm lower heart rate, with much greater bonk resistance and much less tendency for my legs to fade as the miles go by.

For someone who wants to do a double century or similar long distance event, but specifically in a group setting with pack of riders, they need to care very much about their z4 and 5 power, so they don't get dropped on the hills, etc. So they might still want to have a z2 based training plan, but they need to do a couple 4-week blocks of different length intervals as they approach their event. And maybe even do an abbreviated interval workout every 10 days or something like that during their build up / base phase.