You can plan and prep, you can hop on and hope for the best. There is no right or wrong way to thru hike. in regards to the AT, you have plenty of time to get rid of gear/buy gear while on the trail. You have plenty of miles to get you in shape. at the end of the day, you are either going to make it to the end, or you arent. no amount of prep and plan is going to change that.
#1 mistake of beginner mistake - Thinking there is a way you have to thruhike.