Apple Watch exercise goal

To earn 1 minute of exercise credit(green ring);

Travel a full minute at a brisk walking pace(3.3 mph approx) or above.

Raise your heart rate to approx 75% of your maximum.

When you're at around or above this heart rate for the entire minute you'll receive 1 exercise minute credit.

If you pause or slow down then you won't get any exercise credit.

For 15 minutes of your walk you were not working hard enough.

You need to get and keep your heart rate up in order for it to count as exercise.