Thanks for writing with your questions. I’ve probably answered something similar somewhere but tracking it down would take time and it’s better if I just address exactly what you are asking. The old Mailbox trail averages almost 27% for 5000 vertical feet making it ideal training terrain. Very few places close to big cities offer so much relief and with such a steep trail.
I suspect the issue for you on this ME workout was that the pack was too light. We call these Z3 efforts because they’ll feel like Z3 in your legs and have the same training effect. Your unweighted Z3 climb should feel limited by the whole package (global fatigue) whereas this one is meant to target the legs. Try 40lbs next time and see if that does not lower your HR.
If you are doing ME workouts I would drop the any other high intensity work like the Z3 uphills. Since you are unfamiliar with this type of program there is way more risk of over doing the high intensity than under doing it. One hard ME a week will do the trick. Keep the volume of Z1-2 high during this time as you will need to aerobic support.
I hope this helps,