Congratulations on adopting your dog; I hope you have many pleasant years together with your new hiking buddy! You might want to search for previous dog threads--there have been a number of them.
Definitely start with short hikes and work up slowly. The same is true for a carrying a pack. Start with an empty pack and add weight grdually. Hopefully the shelter or your veterinarian can give you some idea of your dog's age. An immature dog should not be going on long hikes or carrying a pack, which can cause permanent joint damage. With larger breeds, especially those prone to hip dysplasia, that's close to 2 years old. The conditioning will also help toughen his pads so they won't get sore on longer trips.
Remember that a dog can't sweat, except through its pads, and otherwise can cool only through panting. Heatstroke is a frequent problem with dogs, and it's a veterinary emergency when it happens. To avoid this, he'll need frequent drinks of water and frequent rests to cool off in hot weather (for a dog, that's anything above 75*F). Try to avoid high, exposed trails in warm weather and look for shady trails with frequent water sources.
Training and socialization, training and socialization are the most important aspects of training a hiking dog. I strongly recommend several obedience training classes if you haven't already started. Once the dog is at the intermediate stage, consider an agility training class. If your dog is willing to run through tunnels and over teeter-totters, you will have far less trouble coping with deadfall or with crossing logs over streams.
Remember that many other people on the trail are wary of loose dogs (often with good reason). Also, many otherwise sweet and gentle dogs tend to become protective or even aggressive when in strange places. You don't want your dog chasing wildlife or barking at horse parties, which could cause a serious accident. There have also been a number of instances in which a dog has gotten too close to a cliff edge and gone over. For these reasons, unless your dog is thoroughly trained to stay right next to you and ALWAYS come when called, keep him on leash. I have trained my dog to stay on the trail right behind me, and I put him on leash when hikers, horses, wildlife or, especially, other dogs appear. Letting the dog go ahead makes him think he is the leader instead of you. It also creates an interesting situation if the dog stops right in front of you to check out an interesting smell! I've found that it's far better to keep mine either behind me or alongside in the "heel" position, depending on how wide the trail is.
I take a closed cell foam pad--basically a cut down "blue" or "green" pad--for my dog to sleep on. My dog is a Lab, and with his downy undercoat theoretically should be fine below freezing. However, he is an indoor dog at home, so he wears a jacket on really cold nights or if his fur is wet (to keep his wet fur separated from my sleeping bag).
Your dog may also need booties if you're traveling over rough, rocky or gravelly ground. For snow, clipping the hair between his pads helps, and using vaseline or (better) beeswax may be better than using booties (which cover up the dog's natural "crampons"). In our volcanic areas in the Pacific NW, things like volcanic cinders or lava can cut up a dog's pads in a big hurry, and the booties are essential.
Be sure your dog has all his shots and that you use a flea/tick treatment (Frontline or K9 Advantix) monthly. Check with your veterinarian on health aspects and on first-aid items to take for your dog.