Keeping my heart rate in zone 2 is next to impossible.

Hi Skip! I think it started with getting Covid in Dec 2022. Symptoms were pretty mild and first run back afterwards I was able to trail run 2.5km steady. Then in Feb 2023 I agreed to test out a running plan by a friend at our CrossFit gym. He wanted us to keep our heart rate low and to stop and walk to keep it in range. I think that was a bad habit and now I can’t seem to break it. I am 59, no allergies, probably need to strengthen my glutes and learn how to control my breath better. My Vo2 max used to be 39-41 before Covid and is currently about 33 (according to Garmin). I also stopped taking hormone replacement about a year ago and recently learned my Synthroid Rx may need adjustment. TSH level is almost non-existent but T3 and T4 is going up.