The word cardio is just a term to describe doing an activity that raises the heart rate.
How long the duration and how high the HR be maintained to maintain and improve health is specious at best
IMO
In this day and age we need to spend a small amount of time in the high intensity area and most of our time in the low intensity area.
although walking is low intensity in terms of raising the HR it's the safest in terms of potential soft tissue damage.
if you're using cardio as a means of improving health then make sure lifting is a very big part of it.