I think that if you have that problem, you just need to abandon the idea of mountain bike clipless pedals and either get road ones, or if it's touring, just regular touring pedals with toe clips. You can buy pedal after pedal, but they are all too small under the foot. Even ones that provide a bit broader "platform" around the SPD or other mechanism still seem to concentrate pressure right over that mechanism itself when you're actually using them on the bike, not matter what you do. It's not a concern for mountain biking, but it certainly is a common complaint when we're talking road.

If you resolve the problem with a super stiff shoe, then you kind of lose the "walkable" advantage you wanted in the first place. Keep in mind that with SPD-compatible types, the part of the sole that is in actual contact with the mechanism is a lot thinner than what the sole looks like from the side.

I'm not one who would agree too much with the "move the cleat way back" idea. I know even supposed experts offer that advice, but to me, it makes no sense to resolve a problem caused by poor clipless pedal choice by forcing poor pedaling mechanics on yourself.