One could argue that walking around - kilometers at a time, in the case of getting an egg to hatch - is serious exercise compared to sitting in front of a computer or on a couch.
Not saying walking is not good for you, but walking is very energy efficient. Your heart rate is probably higher reading HN, at least for me. Doing some sprints and other movements, like playing football, or moving the lawn, will give you a good workout though.
If you want to burn fat, walking is one of the best ways to do it. It you want to build muscle, there are better options. But long walks are absolutely good for certain kinds of exercise.
you also burn fat while eating pizza, but you put in lots of more calories. Walking doesnt use many calories. Its still healthy though, and better then nothing at all.