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The real question comes down to what you are trying to accomplish. If you haven't exercised for the past 6 months or are just trying to break an unhealthy cycle, going for a 30 minute walk 3 days a week can do wonders for your health. But what you need to do is keep the body guessing. After doing the same workout routine for about 5 weeks your body will adjust and that workout will no longer be as helpful. So once you have been steadily walking for several weeks, turn one of those 30 minute sessions into half jog half walk. A couple weeks later, add some 50 yard sprints into your jog. Its all about the small incremental changes.
The main point is that any exercise is good! But be sure not to get locked into the same routine of walking or jogging. For me I go jogging only 2 or 3 times a week, and other days I workout by playing pickup basketball or frisbee or soccer. Or in the words of the lovable character Gordon from dodgeball:
"I'm gonna shock the biceps later, then some cardio. Keep the body guessing."