Your not walking fast enough;
To earn exercise or workout credit you must move at the speed of a brisk walk.
To earn 1 minute of exercise credit(green ring);
Travel a full minute at a brisk walking pace 5.3kph or 11.3 minutes per kilometer.
Raise your heart rate to approx 75% of your maximum.
When you're at around or above this heart rate and speed for the entire minute you'll receive 1 exercise minute credit.
If you pause or slow down during this minute then you won't get any exercise credit for that minute,it will still measure your steps and calories burnt but will not give you any credit towards the exercise ring.
Your 7.43km walk should be walked in approx 1hr 21mins
Source;Successful phone call I had with apple support.