That said, as someone who has walked on cobblestone - a modern road in modern boots is different to an ancient road in ancient boots.
I'd bet money the 'real' travel times would differ.
> an ancient road in ancient boots
Boots? That's luxury! Sandals more likely...
In the case of Spain, many long distance roads are no longer safe for pedestrians as there are cars on them now. Madrid to Milan would be a suicide missions in current day.