Adventist Youth Honors Answer Book/Backpack trip
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Read over the answers in the Backpacking and Hiking honor before setting out. Both have a lot of information that is helpful for a trip like this, and a 15-mile trip will meet the "big" requirement in both.
"Carrying all needed supplies" means that you do not hike from point A to point B while someone else drives an SUV there laden with all your gear.