If you've read this far, you probably have a fitness program to get yourself strong -- your trip will be much more fun and safer if you get fit well before you reach the trailhead. All of us work out, alone or together, all year long. As the date of our backpack nears, we take a big hike every few weeks, building to a hike that is about as high and as long as a typical day on the trip will be. We also do one big local hike with full packs (35+ pounds).
On a recent week-long FLAB hike, we were in California's Sequoia Nat'l Park when a large group of college students straggled into camp, shrugged off their oversized packs, and sagged onto logs around their campsite.
We sat and watched in wonder as the exhausted and weary hikers began dug into their packs and pulled out (among other things) a 2 gallon cooking pot, bottles of hot sauce, boxes of graham crackers and lots of whole fruits such as avocados and grapefruit. They were having fun together, but they had not yet tackled the 11,600-foot pass under their staggering, and unnecessary, weight load of 50-70 pounds per person, including even very slight women.
The stronger you make your body and the lighter you make your pack, the more upright you'll hike and the more you'll enjoy the scenery you came to see.