Running burns a few more calories than walking. But weight loss should not be the goal of exercise. To decrease risk of mortality. To increase function. To increase balance of the muscular-skeletonal system.
Any exercise is better than no exercise.
You can't be fit if you're fat. Fitness Fact Number 5 You can be fit and have some fat. It may be dangerous for the cardiovascular system to make it hurt. Fitness Fact Number 6 Moderate intensity workouts are sufficient. Every little bit helps.