Asphalt/concrete isn't either. Gravel or packed dirt is way better.

That said 30km/day is very real, even in mountain country. It takes 6-7 hours, the rest is resting/gawking/laying on a mountain watching the sky (or trying furiosly to find that effin trail on osmand). I walked that a few times a week when I lived in Montenegro, and that was 50/50 roads and mountain trails from sea level up to 700m-ish elevation and back. No fancy shoes, no prep but for a hat and couple bottles of water. After a third walk or so it isn't even exhausting.

Sure, but I'd imagine the average human could walk substantially further on flat modern roads in modern footwear than on cobblestone with ancient sandles.

You use 10–20% more energy for just mildly uneven surfaces - probably more for roman cobblestone.

Actually no. Uniform flat surface is tiring compared to a trail even in best shoes. Try walking 10 km on hardtop then compare to walking same distance on unpaved road.

For shoes we also went not too far: Merrel sandals basically replicate roman caliga with modern materials. And it's not like you can use even halfboots in mediterannean unless it's winter - it's just too hot.

Empirically, yes.

You seem to be equating flatness with surface hardness. Tarmac isn't unforgiving to walk on because it's flat, it's because it has no give.