Where did you get this definition of a "brisk walk"? was it from Apple? That's the kind of info I've been looking for. a 16m mile is an AWFULLY brisk walk and 120bpm is a pretty high heart rate for some people. I'd hope apple watch would at least use your age to calculate your target HR zones...
I also have to add though- I find that ANYTHING I do as an "other" workout, even sitting on my couch, gets added to my exercise ring. (I tested it). Yet the other day I walked for 64m as an "outside walk" and got 4m credit. Then this weekend I tried the elliptical and indoor cycling settings (and I've read of a lot of problems with indoor cycling) and they both worked perfect and matched the hr/calorie stats the machines gave me.
Hopefully this this will all get fixed in a software update soon- I have to expect it will be