You might consider having Valda and Willa bring back even more supplies, since they only have to make the trip one way, once you fully provision yourselves.
The journey back is without incident. I've rolled a wandering monster encounter (because there are ticks and things in the tunnels, but you don't meet anything).
I will need a d20 roll from Engelhart, Embla and Rob.
To calculate the distance that can be covered in one day, subtract the numbers under the columns, "fewer miles ..." from the base distance travelled. Roads ignore subtractions associated with vegetation. Vegetation is assumed to be off road. Both roads and vegetation are cumulative with subtractions under "elevation change."
Elevation change depends on the general nature of the slope. Hills can be walked over; scattered peaks are marked by areas that require scrambling, where the character must use hands occasionally when moving overland. Dense peaks are marked by actual rock climbing, where sufficient areas require both hands and feet to overcome obstacles.
Thus, a group of adventurers moving with 3 AP start with a base movement rate of 20. They enter a subterranean dungeon, equivalent to "path & woodland," and have that movement rate cut by 8 miles. They then descend a distance of 350 feet throughout the day, and have that movement rate cut further by 5 miles.
Result, the party is able to travel through the subterranean at a speed of 7 miles per day.