What counts as "walk exercise" for the green ring??

Does anyone know what counts as "exercise"? i.e. what does it take to advance the green ring?? All the apple watch info I've seen says "brisk walk". I went for an "outside walk" using the workout app. I walked for 46 minutes, and did 2.28 miles, with an average heart rate of 107 bpm. Yet it only counted as 1 minute toward my green exercise ring!!

My resting heart rate is pretty low, usually in the 50’s, so 107 is a significant increase!

Does anyone know what the criteria are? Does your heart rate have to be above a certain threshold?

I don’t want to use other if I walk. I understand motion sensor, my arms both move when I walk.

Is there a specific target HR or HR INCREASE that outdoor walk is referencing?

Im currently injured and unable to run. I can’t close my ring