>>549312In regard to physical status, just be in shape. It takes more mental fortitude than it does physical.
In regard to mental stability, I'd day you need to spend a long time preparing yourself mentally for such a long trip. It can be impossible to complete a hike this big if you don't go into it with the right mindset.
Don't spend too much time alone if you have a history of depression with risk of relapse. Having that much time alone with nothing but your thoughts and a huge path to cover can cause even an average cynic to question things like their significance.
Besides, having company is good. Human beings are nothing if not a social species.
The Appalachian trail is fucking huge, and it's time-consuming to finish, but I say it's better to have the mindset to complete the whole thing. You don't want to get psyched out early and give up, but sometimes it's good to make smaller goals if you think you can't take the whole cake. You just pretend you can eat it all, go one slice at a time, and finish when you can't stomach any more.
Personally, I'm an all or nothing kind of guy, so I'm not sure how to cover the middle ground.. Shoot for the stars and keep moving forward, I guess.
Also, don't forget sunblock or lip balm.