To earn exercise or workout credit you must move at the speed of a brisk walk.
According to apple support, for the activity ring to register your workout (for outdoor walk or exercise ring) you need to maintain a speed of approx 3.3 mph or faster,this is their definition of a "brisk walk".If you slow down or pause the watch will stop tracking your movement as 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 why it is important to calibrate the watch when you set it up from new,the idea being that you're being encouraged to put more effort in to walk faster and to get your heart beating faster, if you just walk at a slow pace then you are not going to improve your fitness or get all those award badges that we all seek.
If you've had the watch awhile and it seems to be inaccurate(eg. you excercise for 50 mins and are credited for only 5) then you can reset the calibration data on your phone and then recalibrate it again by doing a walking or running workout for at least 20 mins,the watch then learns your movements for an unspecified time in order to make the measurements as accurate as possible.
