15 miles by bike and walking on foot. Well Snowdon is 3 miles up and 3 miles down. You get the highest elevation by going up the Watkin path. They mention a waterfall abseil on the way down again, which also suggests the Watkin path? Can't think of any other waterfalls there. As the pick up point is Llanberis, Watkin path also makes sense?
I was looking at the map and if you trace the remaining 9 miles back down the valley toward Beddgelert, then that suggests the mine would be in the area of either Croesor, Cwmorthin or Llechwedd.
As the Snowdon ascent is something like 3000 ft, they are missing the extra 1500ft somewhere. Where do you come out of a slate mine and have to climb so high to get into a vehicle. I could only think of the new road above Llechwedd for the ziplines? ( Croesor and Cwmorthin you walk out and down to your car).