I'm an expert on this, though it's a pain since I only get about a week on the AT a year. I try to stay in decent shape year-round, but once I plan a hike, I totally change my training.

1. Walk in the shoes you'll take on the trail. If you can only go one mile, only go one mile. You'll pick up miles fast. I walk until I can get about five miles--no pack--without being footsore.

2. Load up a pack with the weight you projected you'll carry. I start around 30 lbs. I carry that until I'm not footsore at five miles.

3. Increase mileage and pack weight (mileage is way more important) until you can hike you're longest projected day at least once or twice. I end up training with about 20 lbs. more than I'll take to trick my body. Unfortunately, there's no real simulation for mountains except a mountain.

4. At the gym: any ab or core work. Supermans. Wrestler twists. Stairmaster once a week. Also, RDL's (Romanian Deadlifts) or hyperextensions really strenghten the back quickly. Also, do some shrugs so your traps don't get so sore. As previously stated, lunges rock. I tried training for a hike one time by running. Bad choice. Walk.

5. Find local trails, put on the pack, and go! If someone looks at you strangely, tell them you're training for a long hike. Almost always are they interested and will ask you where you're going. That makes you feel good about your training and that it has a purpose.

BTW, I'm 260 lbs., so getting in shape is quite an ordeal. For a 40-mile AT trip, I'll start training about 1.5 months prior. My philosophy: I get one shot a year to hike the AT: no way is conditioning going to stop me. Weather, perhaps, but not conditioning.

Hope this helps!