stoli, sounds like a fun trip! Here are some tips.
What type of physical condition are you starting in? If you’re in reasonably good shape, start stepping up the intensity of whatever endurance activity you already do. I would consider spending some time on a stairmaster, since you don’t really have any hills! The best way to train for backpacking is… carrying heavy things around. Walk the stairs of buildings with a pack on, gradually increasing the weight. Find out how much weight you will be expected to carry from one of the experienced folks. Remember that backpacking is more about endurance than speed.
Gear
Are you buying your own backpack for the trip? If you have the option and the $, shop around at various places (REI, EMS, etc.) for fit and comfort. Find a salesperson that knows what they’re talking about. They should have stuff sacks with weight so you can load up the pack and see how it feels with weight. If not, bring your own. You really can’t tell by trying on an empty pack. And a comfortable pack is absolutely critical in enjoying the experience!
The same goes for boots. Don’t skimp on these or you’ll be miserable. I use leather boots with lug soles for the ankle support, foot protection and weatherproofing - on a rugged trip, the goretex combos can fall down. Lightweight boots are better for day hikes where you’re not carrying 1/3 of your body weight day after day! Your feet will hurt, but you don’t want blisters or sole bruises from ill-fitting or flimsy boots. You should be able to wear them with a thin polypro liner sock and a heavier weight outer sock - wool or a non-cotton blend. This will help prevent blisters. On a long trip, I will start out by putting a square of duct tape on the back of my heels just in case. After you find a pair, wear them around for a while indoors with the right sock combo - take them on one of your stair climbs, with weight! You may find that they are not as comfortable under load, as it were, or after a few hours. Wear them around the house, or at work. As long as you don’t scuff them up too much, you should be able to return them.
Altitude
This can be a killer. Altitude sickness sucks. Headaches, nausea, decreased appetite, lethargy - not the ideal symptoms for enjoying a physically demanding trip. Also, you just can’t catch your breath! What kind of altitude will you be hiking at? If you will consistently be above 7000 feet or so, ask your doctor about Diamox. This drug can help you adapt. Hydration is critical, as is setting a pace you can maintain.
Attitude
Obviously. It sounds like you’re getting it right from the start by asking what to expect and preparing! Get a suggested packing list. Find out about the route. Do you know the people you’re going with? If you’re the only novice, (even if not) be ready to ask for what you need. Don’t try to tough it out - you could just make yourself unhappy and even more exhausted. Take it easy, especially on the first few days. If you start to feel a hot spot on your heel, stop! If you’re struggling, don’t be afraid to ask for someone to take some of the weight you’re carrying. Above all, be positive. Learn about some of the plants and animals you might see, take lots of pictures, get involved with meal planning and preparation. Food is key.
Feel free to email me with other questions and I’ll answer them in more detail than you want, as usual.