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The Kohechi Trail is split into four sections and on the day Pattie went missing she was on the third section. It's apparently quite common for pilgrims to only walk the final section from Totsukawa Onsen to Kumano Hongu Taisha, missing out the first three sections completely; it has been said that Pattie only decided to walk the first three sections at the last minute, because she was running ahead of schedule, so I'm presuming her original intention was also to only walk the final section.
If the trail turned out to be more tiring or difficult than Pattie anticipated, is it possible she decided not to walk the third section after all? Pattie was planning to meet with friends in Totsukawa Onsen on the night she went missing, so one way or another she had to get there.
I also wonder how common it is for pilgrims to run ahead of schedule on these trails. Most of the trails are split into clear day-by-day sections with pre-booked guesthouses at each end. You should be able to plan out exactly where you'll be on any given day. It's considered rude in Japan to walk up to a guesthouse without booking a room in advance, so it's unlikely Pattie would have veered too far off her schedule.