i agree with the others, plans are made to be broken and nowhere is this more true than the AT. and those who attempt to stick firmly to plans they concocted in march invariably end up miserable and either learn to ditch the plan and write a new one with their new knowledge or they quit.
not that plans are bad, but if you have never done a long distance hike you have no idea what to expect and how you will adapt to the lifestyle. and that is one of the best parts about the journey, the adaptation and the growth.
i would suggest going out with no plan but to TAKE IT EASY in the early going. take days off whenever, stop early, dont push it and dont think about miles only think about your body. 6 may look horrible on paper, and some other hikers may be pushing for 12, but if your body is saying no, listen.
then when you are two or three weeks in you can sit down and write out an itinerary if you need to. it will be much easier and realistic.
if you want an idea, though, i will tell you how the pacing worked out for me. i am 25 tall and strong, although i smoke. but in the early going i was one of the more aggressive hikers. even still in the first week it was 8-12. in the second, into north carolina but not yet to the smokies i did a couple 15s but needed to balance them with 10-12s. by the smokies i was able to hit 20, but again i needed to balance with short restful days. almost every other. the body was still creaky. the creaks did not work themselves out until around erwin, when i could hit 15 every day. through virginia it became less an issue of how much i could do, i could do 18 daily if had chosen, and more of what i felt like. this varys greatly for everyone. some people get bogged down in VA and some respond by doing 12s, some by doing 25s. some people like to rest. some people cant rest if they want to. the journey from then on will be a mental and emotional, somtimes physical, ebb and flow and this will be reflected in your milage.
not until VT does the terrain factor in to your milage. if you normally pull 18s, youll be closer to 15. in the whites of NH youll be closer to 12, even 10 some days. and in much of maine 12 will be grueling although there is some 15 type terrain in there. even some 18, but forget i said that because if you remember and you start looking for it you will get very upset with me.
so i wrote alot there. hope it helps.
milo