Compliments of
gencolson !!!!
Okay, so you're going to be having some ups and downs in your trail. Well, the first thing is to not go overly ambitious. I'd pick a, oh, 15 mile trail loop that you can break up into 5 mile days. Knowing where you're going early is important, so you can train to beat your hardest day.
Next, you want to start with the packback you'll be using and a light load - 10 to 15 pounds is a good starting place - and just walk for an hour or two. Leisurely, preferably not on concrete (that'll be hard on your knees). So if you have some nature trails around, that's a great chance to hit them up. For about three months before you go, you want to start doing this about two or three times a week. As you go, you want to slowly increase the weight of your pack by a pound or so. The heaviest your pack should ever get is about 1/3rd to 1/2 your body weight - aim for how heavy your pack will be, at its heaviest.
Now the ups and downs are harder, I say as a former flatlander. For that, you might need to take a weekend to drive to some place with hills and do day hike with your full pack, about 3 times. More if you need it. You /can/ get a heck of a lot of practice climbing stairs of tall buildings, but don't do this with your full packweight, as that's especially hard on your knees at those weights. Your knees are precious! Baby them now, so you can abuse them later. ;) The stairs stuff you need to be doing with some form of regularity - I'd do that before your nature walks. Why before? A - it'll be the harder bit. B - it's the less pleasurable bit, and if you put it at the end, you might be inclined to skip it.
Get your boots and pack early! This way, they can break in.
And the most important thing - very very most important thing - is to put weight in each pack and try it on in the store!
My other piece of advice is look at women's packs. Some men's packs fit some women well. Chances are unless you can impersonate a linebacker, you're not one of them. The weight of packs are on the shoulders and the hips - two pars of the torso that are obviously different between the sexes. A lot of times, `women's outdoors gear` is just men's stuff with different colours. This is not true with packs.