Do you plan to put your phone in airplane mode? Many wilderness areas don't have a signal anyway, and it makes a huge difference in power consumption. It might not matter much if you're using the built-in GPS though. You should do some tests, maybe a day-hike or two or an overnight using your devices the way you plan to on longer trips and see how much power you actually consumer per hour or per day. Then compare that with the capacity of your device(s) internal battery(s), given in mAh, to estimate how many times they would have to be recharged on a longer trip and what capacity of external battery you would need.
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The journey is more important than the destination.