Be certain that you walk for 30 minutes. Don't walk by distance alone. The important thing is to get your heart moving in its prime exercise zone for as long as possible and the stronger you get, the more stamina you build up, the more miles you'll be able to cover in 30 minutes. When you start out for the first time it might take you 30 minutes to go around a single block. Your heart is getting a fine workout.
Time is important.
Distance is not. As always, check with your doctor before starting any exercise routine. This is super-low impact so it won't hurt joints. If your knees begin to hurt, don't move your legs as far apart during the Jumping Jacks. 100 jumping jacks.