Whether you should run or walk to get fitter largely depends upon where you're starting from. If you're a person who has not done a lot of exercise or perhaps done no exercise, then going straight into trying to run could be quite difficult – not necessarily in terms of your fitness but in terms of injury.
All of a sudden you are putting a lot of extra load through your body, particularly your lower back and your legs, and the musculature that surrounds those joints might not be strong enough to support the body weight. When you run there is something like 2.5-3 times your bodyweight going through your legs every time your foot strikes the ground.
"As their body becomes more tolerant, and perhaps they lose a little weight, their muscles are allowed to adapt. You can then obviously start to build short runs into walks"
It’s great when people get interested in running. But often the reason they get injured is because they start off too fast, too quickly. If we take someone who has been completely sedentary and is probably overweight, the best thing to get them to do is to start walking and to gradually increase the difficulty of the walk. You might start on the flat and graduate to gentle slopes and so forth. As their body becomes more tolerant, and perhaps they lose a little weight, their muscles are allowed to adapt. You can then obviously start to build short runs into walks.
One step at a time: the key to successful running lies in avoiding injuries from over-exertion
If on the other hand you take someone who is perhaps more healthy – not especially overweight, and with no underlying injuries – then starting out running can be absolutely fine. Again this is with the caveat that they don't increase their volume too fast, too soon. Typically what we do as coaches is try and stick to increasing the distance run by no more than 8-10% a week. If you were to begin with 30 minute runs a week, which might amount to 4 kilometres, then on the second week you would increase that to no more than 7 kilometres.
In terms of what is actually better for you, obviously the more active you are generally the better the effects. We know that as you start running a little bit harder your breathing increases, which is pretty indicative of energy expenditure and cardio-vascular strain. What that will do is trigger off a series of adaptations over a longer period of time that will lead to improved cardio-vascular fitness. You will be able to work at a higher intensity than you could previously. Your heart and lungs will work better together. Oxygen will be delivered to the muscles more quickly and effectively allowing you to work longer. That then correlates into improvements in weight loss.