How fun. My Dh and I read "A Walk in the Woods" several years ago and got completely infatuated with the Appellation trail. We added it to the list.

When we are planning a long back pack trip, we do all hikes with a pack. Right now my dh is training to climb Mt Rainier. He has had his pack weighted for the last couple of weeks. He wears it when we go anywhere ie day hike, after dinner walk around the neighborhood etc. I regularly see people at the gym who wear backpacks on the stair climbers, but I live in a pretty hiking common area.
Any overall conditioning would be helpful (mix cardio and weights), practice with weight when you do hike, and have fun.