Fitness Preparation for Hiking

It really depends on his fitness level already.  I take Xerxes on a 2.5/3 mile walk every evening, then after an hour or so rest and some food, we're off to the dog park for 2 hours.  I didn't realize he was in such good shape until yesterday when the neighbor's dog came with us.  He was pooped 1/2way through the walk.
 
I'd start off doing walking over various terrain at increasing distances.  Make sure that his pads are in good shape before you go.  You might want to get a water backpack so he can carry his own water, and I'd consider either a "cool" jacket or a cooling collar.

i would start out hiking on some easy trails. keep the distances short. you dont want him to get overly exerted or bored (if that's possible). i know for people once you have some base fitness, you should only up the quantity of the exercise (ie. distance) by about 10% per week (not sure if it is the same for dogs). so if you start out with an hour hike, next week you could go to 1 hr and 6 mins. that is assuming you and your dog are getting regular exercise. i think about exercise in terms of time more so than mileage, because what seems like a long distance to me may not to you and vice versa. so if you and i go (seperately) on an hour hike, whether the distance is the same, we can get the same exercise benefits from it.

i would also start your dog off with a pack, even if it has nothing in it to begin with. then you can slowly add a little weight. no reason he cant carry his own food and water eventually. []