I did cardio throughout my time on dnp. Though it was difficult, i improved my oxygen capacity immensely. I was trying to improve my level of fitness as well as burn fat. Just make sure you are taking in some carbs.
your idea of cardio is walking kinda slow that will be fine. Much more than that WILL kick your ass. Even on low dose I get winded, VERY easy. Walking up a flight of stairs ends up feeling like I just did a sprint. If you can do it, you da man. But I wouldn't reccomend it. You WILL lose LOTS of bodyfat while sitting on your ass on DNP.