I’m no coach or expert. But speaking from recent experience, I think you may have started with too much weight. This past summer I did my #0 ME session on a stairmaster starting with 40# and completed 1800 steps in 30 minutes. My HR was right below my AeT but still had that solid leg burn. I came into that first ME workout after accumulating a lot of vert in my max strength period. By session #6 I was up to 60 lbs for 75 minutes at that same pace & HR.
Regarding DOMS with uphill ME I didn’t have much compared to the gym based ME which wipes me out for 2-3 days.