1. Sounds like if you reframe the stage-building work as exercise rather than work, you might gain strength and endurance from it. This idea comes from reading Ellen Langers, who wrote about a study in which two cohorts of housekeepers were studied. One cohort was instructed to think of the job as an exercise program. At the end of the study, this group showed improved fitness compared to the control group.

Some exercises that don't require a gym membership:

• Abdominal crunches with and without a twist to add in the obliques. This should "strengthen the core" which will help stabilize the heavy backpack.
• Stair-climbing. Do you live or work in or near any tall office buildings? Run up the stairs. Then run up the stairs with increasingly heavy backpacks. Or run up the stairs two steps at a time. Take the elevator down and stretch the hamstrings. Will folks think you're weird? So what? Tell them you're training for NYC's elite emergency rescue team if they stare.
• Ankles. During regular walks, try to alternate between supination and pronation of the feet, as well as toe-walking for about ten steps each every so often. This is to protect the ankle from twists and sprains, which are likely to occur on rough terrain.
• Breathwork. The only thing I took away from "Chi Walking" was to mindfully synchronize my breath with my steps. I might breathe in for two steps, hold for two, breathe out for two, hold for two (which is essentially box breathing). Then go from two steps per breath phase to three, then four. This should strengthen the breathing apparatus, stretch the rib cage and improve breath capacity. The walking is done at a comfortable (meditative) pace, IIRC.