I know that when I run half a kilometre to catch the bus, I’m tired and out of breath on arrival, and sometimes the muscles in my legs hurt, whereas patiently walking the same distance has no such effects.
I don’t believe people who say this. I don’t believe running doesn’t use up more energy than walking, and I definitely don’t believe it uses up less.
Isn’t it a law of physics that it requires more force (energy) to move an object very quickly than to move it more slowly against the same resistance? More acceleration requires more force, for starters, but doesn’t the air resistance drag also become larger at higher speeds?
Also consider that when you run, not all your energy goes into moving you straight ahead. Every running step is a little jump, launching you upwards as well as forwards, isn’t it? When walking, you don’t need to overcome gravity, except the minimal effort it takes not to fall down.
Furthermore you don’t do cardio only to burn calories, you also do it because the heart is a muscle that needs training, in order to improve your stamina and decrease the risk of heart problems. Walking doesn’t get your heart rate up very much, so as cardiovascular exercise it’s not very much use.
If you’re worried about your joints, take a bike instead and ride it like you’re late for something important.