We would have (and could have) traversed the underground passage in the space of the daylight hours. But we had some unforeseen obstacles and equally unforeseen discoveries, all of which decided us to remain down here for awhile.
First, our progress was impeded and slowed down by a blockage. A section of the tunnel ceiling (a floor from the structure above) had caved in and all access around was cut off. We were at the point of turning around when Elijah found an entrance into a building. We entered and by trial and error made our way up and over the obstruction.
It was all providential. Our delay meant that we encountered a band of people that needed our help.