[bryanlarsen]

The study specifically includes walking in their list of exercises. You probably do want a mix of exercises -- 90 minutes of vigorous exercise a week is a standard recommendation, but 10h a week of vigorous exercise is probably contraindicated for the average 57 year old in the study.

Walking is good exercise for anybody who is out of shape.

[Fire-Dragon-DoL]

Walking as moderate exercise is not "just walking" though. You need sustained speed