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.
You use 10–20% more energy for just mildly uneven surfaces - probably more for roman cobblestone.
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.
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.
I've walked for 8 hrs before, wearing modern sneakers on roads with vehicular traffic, and travelled about the same distance estimated by Google Maps. I'd estimate the Roman would like be able to travel in the same duration, barring other extenuating factors (like Caesar staging a revolt against the Senate and crossing with his legion right in front of your planned route), with worse shoes but more walk-friendly roads.