For calorie expenditure, running beats every other form of exercise. Even at a leisurely 10-minute-mile pace, an 11-stone man burns 700 calories an hour.
When walking, one foot is always in contact with the ground, so the workload is easier - the same 11-stone man would take twice as long to get the same calorie-burning benefits from walking.
High-intensity exercise such as running can suppress your immune system. Research on marathon runners suggests the higher your mileage and more quickly you increase it, the more likely you are to get sick.
Walkers get ill less often than both sedentary people and runners. In one study, they experienced half as many colds as non-walkers, and showed an increase in the number of cells that attack viruses.
Running impacts on the four big causes of heart disease by lowering blood pressure, reducing the risk of diabetes, raising HDL cholesterol and helping to shed excess body fat.
One study found that women who walked briskly for at least three hours a week had the same protection against heart disease as women who exercised vigorously for 90 minutes a week.
For those unaccustomed to it, the effort involved in running can increase stress levels and anxiety. But in the long term, studies show that running can make you more resilient to stress and depression.
Moderate exercise is thought to be more effective in alleviating anxiety than high-intensity exercise. In fact, one study found a single walking session reduced tension as effectively as a tranquilliser.
The impact involved in running can increase stress on the spine and raise the risk of lower-back pain, especially in runners who don't use good technique, and who have limited flexibility and poor core stability.
A recent study found that three hours' walking a week was more effective than specific lower-back exercises in reducing pain, improving mobility and lowering psychological stress in back-pain sufferers.