Paul,
It’s high, but not unreasonably high. However, it’s possible that our 80/20 HR zone system may have an upper accurate limit. For example, your Zone 2 is now 160-177. 160 feels so high for Zone 2, but it may not be a problem.
I would validate this against Pace and consider using Pace instead, which I’d recommend independent of a high LTHR. What was your average pace for that same 20 minutes? Plug that into the zone calculator and get a set of Pace zones. Then, run at 165bmp (low Zone 2) and see how matches to Zone 2 Pace.
That’s more of a act of curiosity, I’d still switch to Pace as your primary measure regardless.
David