I would also say, don't plan too much, because most of the planning you do beforehand usually gets thrown out the window within a couple weeks.

Also, maildrops are not needed on the AT at all, but if you do use them, they take minimal pre-planning. If you have someone mailing them for you, you can just give them a call a couple weeks before you get where you want the drop and have them send it.

Food can be bought all along the way with no problems, unless you have special dietary needs.

Most AT hikers I know, including myself, would just plan out a week or two in advance, and then adjust that plan as needed.