While any form of exercise if good for the body, there are two main differences between walking and running on a treadmill. The first is calorie usage. Running on a treadmill uses far fewer calories than walking over the same amount of time. It has been shown that running for an hour can cause you to lose over 300 calories more than walking for an hour, leading to faster weight loss. The second difference is the bodily impact. Running has a higher impact on joints, bones and muscles than more moderate walking exercise. This can be particularly important for elderly people or those with fragile bodies. The important thing is to find what your body is comfortable with and be consistent, because any exercise is good exercise.