I want to get more exercise. Therefore I should walk rather than bike.

This is about transportation. Walking doesn't get you far. A bike uses similear energy but you get farther.

Walking covers most of my journeys for which a bike would work. I am not going to bike a hundred mile, or even 15 and back. Nor is it practical to bike to the supermarket. On the other hand its easy to walk a few minutes to the local shops and pubs.

Transportation and exercise are linked. Walking kills two birds with one stone.