Maybe you've heard the old saying: the best way to train for backpacking is to backpack. If you hadn't heard it before, you have now. And it's a pretty solid statement. Nothing can prepare your body for backpacking except, well, backpacking.
For anyone who is interested in starting backpacking, my advice is this:
1. Invest in a good pack that is comfortable for you and fits your body type.
2. Get yourself the proper gear such as a sleeping bag, a sleeping pad, a tent, a stove + fuel, a water purification system, food, warm layers, rain layers, headlamp, etc...
3. Then pack up all of your gear into your backpack, strap in on your back, and hit the trails.
Day hikes will do. Just get that full pack on your back whenever you go out. You don't want the first time that you hit the trail with a full pack on to be when you are heading out on your maiden overnight backpacking trip.
Trust me, wearing your loaded up pack on day hikes is the BEST way to build up the proper muscles and strength required for backpacking. All of the squats and burpees in the world won't be able to prepare you for a steep climb up a mountain over uneven terrain with a 30+lb pack on your back.