It's more efficient if you're time constrained. However it's probably worse for untrained and overweight people due to higher impact on the joints.

For improving cardio? For sure. It is a very time efficient way of increasing your cardio. For improving back health? Not really, studies indicate that walking and running give about the same benefits. Obviously if you are unfit enough you need to walk before you run or you will just have a miserable time.

For people that experience the runners high, this probably doesn't happen with walking

It depends on the individual and their condition and ambitions and circumstances.