Stick to the Mediterranean countries - Italy and Spain are good options as the temperatures are perfect in october. A couple of specific options are:
Komoot and Alltrails are clearly the two most popular with our customers, they're pretty intuitive from what I know, and they do have pages on their websites with instructions on how to add gpx files then follow the route. We can also provide links to the relevant pages on request. As for battery preservation, generally not an issue with modern smartphones, but it's wise to take a power bank as a backup. I might also mention that airlines are getting quite strict about power banks because of the risk they pose in storage, you have to carry them on board not in the hold, and may even need to keep them at your seat rather than put them in overhead lockers.
Please do also note that these will only be used as a reference point, as our walking notes are so detailed, you shouldn't need to always be on your phone. GPX routes are used as a reference point and for further reassurance if you are ever hesitant.
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