How did you determine the top of your Z2: AeT drift test, nose breathing AeT test, MAF, or 75% of maxHR?
(As an aside, I would simply suggest jogging on flat or gently rolling terrain for say 2-3 months at about a 10min/mile pace, 3x per week, working one’s way (quickly or slowly) from 20min to 60min per jog. I would finish each jog with about 4x10sec uphill sprints. You can record your HR if you want for these jogs, but I wouldn’t adjust my pace to target a certain HR–it should just feel moderate/medium in effort. Then after about 2-3 months, I would test and apply specific heart rate zone training zones, and begin a training program if you want. This approach allows for a certain baseline of fitness, running economy, habit formation, enjoyment, injury resilience, and anaerobic power–all to be developed without any significant risk of overtraining. A simple strength program could also be performed simultaneously–anything from the NY Times 9min work-out to the UA general strength routine would be fine depending on your time and goals. A weekly half-day hike would be great too.)
just my opinion of course,