If you have ADS and did the AeT test, prefered: https://www.uphillathlete.com/heart-rate-drift/, alternative: https://www.uphillathlete.com/8-diy-steps-to-figure-out-your-aerobic-threshold-indoor/, or lab testing. Then all your runs should be bellow, or at your current AeT (which is top of Z2 in the UA zone system). Your HR can drop e.g. in the downhills and that’s no problem. As far as i can say you just don’t should exceed it for a long period of time.
i asked similar questions maybe you find something useful here:
have a great day — mirko