In my experience when I was recovering from injury, an outdoor walk did not record green ring minutes unless I was walking faster than about 19:15 per mile. (It's hard to tell exactly, but I had almost all green ring minutes on a walk that had most miles about that fast or faster, but when I was first starting out, and I was going about 19:30 to 19:45, I had very few green ring minutes.) It's possible that everyone is different, though, based on age, height, weight, and heart rate (though heart rate only if you are doing a workout 0 it doesn't use heart rate as an indicator of effort unless you are doing a workout.)