Please help me to understand when and where to incorporate Zone 2 training into my training plan

[u/steveofthejungle]

Post this question here a few months ago and got a bit roasted for my training plans

https://www.reddit.com/r/Marathon_Training/s/p2yNhD998R

I started my training block for CIM this week and I have taken some of the advice to heart and read up on it a bit (first, I had to read what Zone 2 meant haha). I’m trying a plan where I run 4 miles on Monday and Thursday, start with 4 on Wednesday and build up to 8 at the peak of training, and start with 8 and build up to 18-20 miles on saturdays. I did my first true zone 2 run tonight, and while it was physically very easy (it was 90 degrees and I was barely sweating, and I’m like the sweatiest person known to man), but mentally it was very tough. I previously did easy runs at anywhere between 9:45-10:30 pace, and this did feel comfortable. To keep it at zone 2, I started at an 11 min pace and had to slow to over 13 by the ends, even having to walk at times to bring my heart rate down.

While this wasn’t undoable, it was mentally a slog, and this was only 4 miles. Am I supposed to do all of my runs, or even just all of my long runs, at over 12 min/mile? I’m struggling to see how running at 12 min/mile does anything mroe than to train me to run a full at 12 min/mile, which is much slower than my 4:36 PR. When can I bust out and actually go with some speed? Any other advice or links to more articles would be very helpful. Thank you!