If you're largely sedentary, getting an hour of exercise per day doesn't counteract the fact that you've been sitting for 12 hours per day. [1]
Simultaneously, walking slowly all day does not burn many calories compared to a proper workout. But that's fine, because the goal of walking slowly all day is not to burn calories, but simply to keep you from sitting all day.
It seems to me the best approach is to simply do both.
If you're largely sedentary, getting an hour of exercise per day doesn't counteract the fact that you've been sitting for 12 hours per day. [1]
Simultaneously, walking slowly all day does not burn many calories compared to a proper workout. But that's fine, because the goal of walking slowly all day is not to burn calories, but simply to keep you from sitting all day.
It seems to me the best approach is to simply do both.
[1] http://annals.org/article.aspx?articleid=2091327