Is walking exercise?

Let me say from the outset that I am one of those lucky people who are naturally slender. I’ve always been active, even back when I didn’t play sports or set aside time for exercise.

However I cannot understand walking for exercise. Maybe that is because I am naturally impatient, but the only time I have felt fatigue from walking was as a teenager, hiking 20+ km per day for a weekend with an 18kg pack. So when I see people walking around the block in exercise clothes, I wonder if it is doing them any real good, or if it is all placebo and feel-good.

So have you ever seen exercise benefits from walking? Has walking helped you lose weight, or improved your cardiovascular health? How long and how far did you have to walk to receive these benefits?