I’ve spent way too much time reading the forums here so I’ll chime in with what I’ve learned so far:
Keep under zone 2 (under 130) even later in the workout.
Steep terrain is good because it mirrors your event. I would probably do a mix of flat and rolling and then make weekend trips to climb in the mountains if I were living where you do. But maybe you might have to add in some inclined treadmill or stairwell workouts to get more vertical in if those MTB trails are really gentle. I admit I tend to gloss over those flatlander posts because I live in the mountains.
Don’t bomb downhill, just go Z1 and count the entire time towards the workout. (if it calls for 2 hour Z2 uphill workout, count the entire time up and down towards the 2 hours). Although sometimes I like to jog downhill because otherwise my HR goes too high, still. (I think living at elevation is part of the issue too, our AeT is just lower here).
When I started following TftNA’s plan in the fall I was ridiculously slow too. I was cross country skiing and hiking uphill in what felt like slow motion. I’ve definitely sped up.
I did an AnT test at the beginning and it is helpful to know if you are aerobically deficient (so if you can add in the occasional Z3 workout or not) and I use Training Peaks so they use that number to calculate your training stress as well. I’m guessing based on your previous post about AeT that you probably are aerobically deficient though (the fact you could work out at an hour at a much higher HR in your initial test.) But you won’t know when you’ve fixed your ADS unless you know your AnT.
One more note that I wish maybe I had read when I started — I personally have found the most satisfying part is going from carb burner to fat burner. I was the kind of person who always had a Gu or Clif Blok handy just in case (and of course plenty of high carb snacks), or who would bonk on a long day, or just get really irritable when hungry. Now I can go for hours fasted with zero carbs and I don’t get hangry anymore. I started with fasted training then a few weeks ago went high fat low carb to help my training, and see if it would fix my allergies. It is helping with both although the first week or two was tough on training.