I have hiked extensively on the AT in Massachusetts and the White Mountains of New Hampshire. A through hike of the entire trail shouldn't take a year -- more like 4 months. Still - it is hard to get that much time off. I'm planning a long trip on the AT in September -- about 300 miles on the southern end of it.
When I hike the hilly terrain of NH, weighing approximately 200 lbs, carrying a 25 lb pack, I burn about 5000 calories per day. It is hard to eat that much -- you have to do it with cookies, crackers, candy bars, beef jerky, etc. All the high-calorie, high density stuff you now avoid! If I am only out for a few days, running a deficit is not too big a problem.
Last week I made a quick two-day trip to climb The Bonds in the Pemigewasset Wilderness of New Hampshire. I hiked 19 miles over the two days and climbed six 4000+ foot summits. About 1/2 of the trip was on the AT. I carried 6000 calories and ate about 1/2 each day, but I felt great. You will also want to consider electrolyte replacement (tablets you put in your water), if you are drinking large amounts -- like a liter an hour. I probably lost about 4,000 calories total for the trip or about 1 lb on the scale.
A longer distance hike would require more calories, but you will inevitably lose weight anyway, so don't worry about it too much. Calculate your needs and do your best. I use beef jerky for protein replacement. I like to camp cold -- but you may want to bring a stove and cook.
If you want to learn all about hiking the AT, check out White Blaze:
Is very active, kind of like OH, and you can find the answers to most of your questions just be reading old posts.