Unless you use "Other" or "Yoga" workouts, I have heard that, according to Apple Support, for the activity ring to register your Outdoor Walk workout you need to maintain a speed of approximately 3.3 mph or faster and increase your heart rate to approximately 75% of your High Heart Rate (from the iPhone Watch > Heart app). This definition of a "brisk walk" is to encourage you to put more effort into the walk, (For an Outdoor Cycle its is 9.8 MPH (or 16 KPH) and 75% of your High Heart Rate.) If you slow down or pause, the watch will stop giving you exercise credit, It will still measure your steps and calories burnt but will not give you any credit towards the exercise ring or workout app. A brisk walk speed is different for everybody as the watch takes into account your age, sex, weight, and heart rate which is why it is important to calibrate the watch and have the correct details set up in the Health app,
I can't vouch for any of those specific numbers, but I'm sure there is some kind of criteria you have to accomplish for a "minute of exercise" to be counted as an "exercise minute. See the "Make sure that you earn Move and Exercise credit" section in:
Get the most accurate measurements using your Apple Watch - Apple Support
You should also make sure your watch is calibrated:
Calibrating your Apple Watch for improved Workout and Activity accuracy - Apple Support
