I have read both TFTNA and Training for the Uphill Athlete. I’ve always been going off of breathing through the nose as my go-to gauge and figured I was well within zone 1 or 2 as I felt like I had more room to go faster without needing to breathe out of my mouth. But I just did a drift test with a HR monitor, and while I was comfortably breathing out of my nose, the drift test came back such that I was in zone 3 (not too big of a shocker though as I was starting to feel the effects after about 45 minutes.
Sounds like “Otherwise” can relate, but curious if others too have noticed this in zone 3?