You're going to hear conflicting advice when it comes to preparing, physically, for a long hike on the Appalachian Trail. Some people will say, "Oh, the trail will get you in shape, so don't worry too much" while others will insist that you begin a strict training regimen of both intense exercise and diet. Both have valid points in some ways. Nothing will prepare you for hiking all day, every day, except doing it, so in a sense, the first group is correct in that actually being on your hike is the most effective way of getting better at hiking. Conversely, being in the best possible shape you can achieve before your hike certainly isn't going to hurt. The entire hike becomes easier, both on your body and on your mind, if climbing those mountains isn't as hard because your cardio is good, your legs aren't getting sore because you've been working them hard (running, hiking, biking, lifting weights, etc.), and you aren't having to feel as weak and tired because you spent the six months leading up to the hike losing 20 lbs off your belly's "baseweight". There isn't an objective answer to this question and how you choose to physically prepare for your hike will not be determinative to your success or failure, but doing anything you can do to make the hike easier can only help, so here are a few basic physical preparation tips before your hike.
Carrying less weight is always good, so if you're carrying around some extra weight, consider a weight loss routine. People will spent $1,000 dollars on a few pieces of gear to save 3lbs...just saying.
Get physical. Hiking the Appalachian Trail is going to beat you up. Being in better shape will minimize that buttkicking. It will still hurt, no matter what, but I promise that it will hurt less if you’ve been exercising and pushing your body before you begin. Your best bet here is hiking, but any physical activity that gets your muscles working is going to be beneficial.
This sort of ties into the first two: go for long walks/hikes while carrying your backpack. You'll need to adapt to how it feels carrying weight on your back, so get a headstart.
I wish there was more to it than this, but this really is basically it. If you're about to embark on a months-long endurance event, you're going to be better off getting in to the best shape you can reasonably manage beforehand. Even if you devote your life 100% to physically preparing, it still won't get you into the shape the trail will, but getting a headstart is going to pay off tremendously.
Nashville Pack and Equipment Company is an ultralight backpacking equipment manufacturer that strives to get you outdoors. With a focus on customer service, innovative and affordable gear options, and environmental stewardship, Nashville Pack hopes to improve the accessibility of America's wild lands.