The final night's fire used hewn wood, which they couldn't have unless perhaps previous campers brought a load with them that they left in a waterproof area.
When Abby suggests going 'off-trail', she says there is a camp ground no more than 'a few miles' away. After a full day walking, she tells Sam the camp ground is maybe another day's hike. Two fit people should easily cover twenty miles in a day. That would make her 'few miles' somewhere over forty.