I would slowly increase the miles if you can find the time. The quickest way I've found to prepare for climbing with a pack is to load the pack and do stairs. I did this through March preparing for the Grand Canyon and it worked very well. We had no problem doing the river to rim in a day. My training varies for each intended long hike, it seems. Some we've trained for distance, others for large elevation gains. The AT has a combination of the two. Nothing beats the real thing for training. The Roan Mt. area (Tn/NC border) has some beautiful hiking without tremendous ups and downs. Perhaps start with a section of that nature. Do some reading on lightening your pack, too. Carrying less weight will help the back issues and make hiking far more fun. Don't be afraid to ask questions here about gear. This is the best forum I've seen for getting good, honest, helpful answers with no snark and BS.