update:
shortly after the OP, I took advantage of an opportunity to run “down the hill” in Denver (my home and usual running routes are >8500′). I had little difficulty staying in HZ2 at that elevation for about 45 minutes
since then, I’ve made it a point to run in Denver or Boulder at least twice a week and my form is improving notably. Also, I’m now able to hold HZ2 running > 8500′ for 2 – 3 miles (until I hit a hill.
plans to do the heart drift test next week with a friend, probably somewhere around Boulder.
thanks all for the advice