I agree with everything Kathy said.
The key is to get your heart rate up. If 15 minute miles (4 mph), which is a pretty fast clip for a walk, isn't getting your heart pounding, you might start jogging a little. But if it takes you 25 minutes to walk a mile and you are pretty winded, stick with walking. Either way it is all great exercise, especially when shared with a friend (human or canine)