The ducal couple made it to flatter ground with rolling hills that made up the mountain pass from Letum to Montville. While they wouldn't make it to Montville that day, they would make it to a halfway point which was a military outpost responsible for monitoring the status of Letum from within Montclair's border.
Men were constantly switched out so they wouldn't overstress themselves after having to see monsters regularly.
When Desmond told Leonor there was little threat going towards Letum when he retrieved the items from the battlefield, he wasn't lying. A couple of dozen monsters weren't an issue compared to the hundreds they would witness at a time in the north.
Even if it felt dire at that moment, it was only a small hurdle for their overall journey. The urgency was there because of Desmond's precious cargo in the form of his wife.