Part of the answer to this interesting question is how good is your maximum volume of Oxygen (O2) per minute of exercise, or what’s your Max VO2. That is because fat can only be burned (metabolized) in the presence of O2, or what is called an “Aerobic pathway”. Bottom line; in your body, walking burns more calories from fat, while running burns more total calories, but with more of those calories from carbohydrates! But as I mentioned above which gas tank (fat or carbohydrates) you are burning from depends on your cardiovascular level of fitness, (your ability to bring in O2). People with a better Oxygen uptake, (Max VO2) can provide more O2 regardless of walking or running and therefore burn a higher percentage of fat as they walk faster or even run.
While walking burns more fat and is good for endurance, running makes you stronger and improves your heart health even more. Both contribute to having good heart health and reduces your risk of heart disease, heart attacks and cardiac arrest as well as looking and feeling better!
So, which is better? The answer is; they are both really good for your health. But ultimately “which one is better” depends on what’s the goal of your workout on any given specific day. If the goal is to burn fat that day, then walk for longer duration and burn more fat. If the goal is to strengthen your heart and build muscle and speed, then walk/run faster for a shorter period of time while burning a higher percentage of carbohydrates.
But, remember to consult your physician prior to starting an exercise program!