Shimano MTB shoes with SPD cleats. One pair is circa 1990, the other is a recent model. They walk fine and the efficiency of clipless is like having a supercharger, compared to platform pedals or loose clips/straps.

The following is a daily occurrence. Some guy half my age is pedaling his too-cool-for-school single speed track bike with platform pedals and groovy sneakers. I'm in my clipless pedals, in the same gear as him, we're doing the same cadence. We come to a grade. He has to stand up and huff-push-huff-push. I just switch on another set of muscles, one leg pushing and the other pulling. The pulling leg may only add about 30% more power, but that's enough for me to glide past him, seated, no downshifting. Of course, etiquette demands that you keep your upper body completely still so that there is no outward sign of any effort at all. Whistling "Que Sera Sera" is always appreciated.