Strolling is recreational, like walking miles around Siena to see the sights with stops for coffee or beer, or to check out the truffle shop. However, in my view the goals of fitness are to be good with recreational and general functionality.
When does walking become a form of endurance (light-moderate "cardio") exercise? That depends on speed and incline. However challenging hiking might be (hills, rocks, etc.) it is still mostly a recreational and visual experience. Even for mountain hiking, most of us need to slow down.
As a rule of thumb, walking is not exercise unless there is sweat and a little shortness of breath. Some people say that if you can have a conversation, it is not exercise. Makes sense.
Here's a good synopsis about walking as exercise.