@rbrbrb2 I started HR training on a treadmill in March. I'd been running since 2015 but read 80/20 as part of a coaching certification so I thought I'd try it. I am sharing my stats to make others feel OK about theirs! Before this, most of my easy runs were 4.8-5.2, and my 10K pace was under 11 min/mi.
March: Zone 2 was 3.8 on the treadmill with walking breaks every 5-10 minutes. I stopped running outside bc I couldn't keep my HR down. I didn't do any zone 1, and did some intervals here and there. I ran 3-4 times a week.
August: started training for a half, doing a more structured 80/20 plan (5 days a week, lots more speedwork than I had ever done before) that I modified. Zone 2 is now 4.2-4.3, zone 1 is 3.4, no walking. My zone 2 HR doesn't spike anymore and I can run outside in zone 2! My speedwork gets faster and easier. My legs NEVER hurt, unlike every other time I've trained for a half.
All of this to say, IT TAKES TIME and you have to go slower than you want! I ran a lot before I started this and it was really hard in the beginning. Give yourself some grace and patience. If you stick with it, it's really worth it!