I feel you would get less work done running or walking on one of these, and you would also not get much exercise value out of it.

It would be better to take breaks often, and every other day take a 1 hour break to do a proper set of exercises in a gym, playing a sport or just running or biking outside.

There was a study I remember seeing referenced a while ago (I think it was in an article about the Mayo clinic guy Levine) where the big point was that if you spent essentially most of your workday sitting, then an hour at the gym doesn't undo that. With this approach I can walk 3 - 6 miles while working. I'm not trying to get my heart rate up to cardio levels obviously; I'm just walking. And I'm not saying I'm a role model for perfect health, etc...