I'm starting a NOBO thru in May and have been creating some physical training resources for the AT.
My background: 6 years US Army Infantry, NASM certified personal trainer, CFL1, and I've owned a CrossFit gym and have spent about 2,000 hours personally training clients.
Caveat: I have never done a thru-hike. I have had lots of outdoor experience and we've done some fairly lengthy rucks in the Army, the worst of which was a 12 mile EIB qualifier where my ruck weighed 68lbs not counting my weapon, FLCK vest, or helmet.
Here's a three-month detailed training plan I put together. It's pretty intensive and not designed for a beginner. For someone completely new to fitness I would highly recommend taking some sessions with a personal trainer and learn how to properly perform everything and have them build you a program tailored to your fitness level before jumping into any program found online. .
Here's a quick list of 7 training tip for anyone prepping for the AT. If you don't want to go to the gym I really recommend buying a kettlebell. You can get them really cheaply and with FREE shipping on Amazon. Best fitness investment you can make. If you just had a KB and did a few sets of goblet squats and KB swings everyday for a few months you'd be in pretty great shape for hiking the AT.
I've been transitioning from a pure strength program into a hybrid strength/conditioning with a lot more high rep workouts. Just did a brutal 100 rep of squats with 80kg yesterday. Can barely walk today lol. Video: https://www.youtube.com/watch?v=GysdiJrHhnI
I plan to keep creating more videos to help those thinking about hiking the AT. Feel free to ask questions I'm happy to help out however I can.