Nothing will prepare you adequately other than hiking on trails with a pack. Before I left for Springer this year, I was hiking a 3.6 mile trail, which was pretty difficult in some sections, about 4 times a week with a 30lb pack. Once in a while I would double it up (it was a loop) and I did not use trekking poles because I wanted to build up my knees - this worked well.
Also, I was hiking the 3.6 miles in 65 minutes. After a few weeks I noticed a big difference in my legs, I had lost some weight, and although I never got my time down to under one hour it just got easier and easier. I did this for about 6 weeks. The strain and recover method I implemented meant my body had time to rest, I remember not hiking for 4 days once due to knee pain, this is a great luxary with training, something you cannot do on the trail generally.
When I left Springer this year I averaged 17 miles per day through Georgia, other than the expected blisters I had no problems, and I smoke! I did something like 23 into Franklin and 21 out, and that's pretty early into the trip. I was very suprised how fit this got me, and it was only 3.6 miles per day.