I am a late 40s triathlete. I have been a runner since the age of 16.

In my dotage I have tried to incorporate HR training rather than just running "all out all the time."

I have read books, read the forums, consumed the internet, all in an effort to understand how to set the zones.

I have seen primarily three tests:

LTHR = 95% of average heart rate over 20 minute max effort (following proper warm-up of 10-15 minutes)
LTHR = average heart rate over last 20 minutes of a 30 minute max effort
LTHR = average heart rate over a 30 minute hard effort

These are going to yield really different results, including because heart rate ramps up. In particular, 95% of a 20 minute test is going to be 5% or more lower than the other tests, by definition.

With the "95%" test I get LTHR of 156. With "100% of 30" I get 162. With "Last 20 of 30" I get 166.

My lived experience is that during a hard effort after ramping up for about a mile my HR levels in the mid 160s. 150s is cruising. Low 160s feels like something I can maintain for a while. 165 or so feels like a "red line" that I can maintain for a good amount of time but not forever. If I push over 168 I have to fall back pretty quickly.

My max observed running HR this year is 186 (during run portion of a tri).

I would be grateful for any guidance from the forum about which of these tests (if any) is the correct one!