hold a given power, rather than by my HR. Zone 1 feels way easy (like a waste of time easy, but feels nice between intervals and when recovering). Can hold zone 2 all day, while talking, but still feels like work a few hours in.

Zone 3 I can hold for 2-3 hours, but I'd be trashed. Zone four I can hold for 40 minutes to an hour, but really wouldn't want to, and would need some recovery. Zone 5, Vo2max, I can hold for 5-8 minutes, and could maybe do a few repeats of that, but the after effects are leg soreness. Zone 6 I can hold for 30secs to a minute, and repeat maybe a dozen times before I'm totally trashed.

After a few rides sort of benchmarking your power, your heart rate zones seem to distinguish themselves.