As soon as the snow clears, I try to get in as much actual hiking as possible. Not big on packing weight just for training sake, but I will carry whatever the trip calls for...on day trips that isn't much, maybe 3L water, raingear and snacks, but on summer backpack or combo packraft trips it can be up to 60lbs of stuff (keeping in mind that 70 lbs has been about my max carrying capacity, at least thus far, but that is over half my body weight). I don't mind carrying heavy weights long distances, but haven't felt the need to stress my knees unnecessarily just to train. I guess I must get enough Training in with weight just doing our regular activities, because I don't usually have much trouble by the time August rolls around.
On days that I don't have several hours to climb something, I try to get in at least 4+ miles on an uneven trail around our neighborhood...I can usually walk/hike it in around an hour. I also bicycle or lap swim once in awhile to mix things up, as I get bored easily. In the winter I walk at a 10% incline for an hour at at least 4mph. A previous lower leg injury discourages me from running, but I can walk quickly without problems.
You will all probably laugh, but one of the best Indoor activities I have found to train for hiking is step aerobics. I like it because you are actually climbing, and it builds similar endurance to what i need in steep terrain. I have taken classes in-person in the past, but find that doing videos or using the EASports Active program for our Wii works just as well. 35 minutes with a 12" step, and I am breathing hard and sweating up a storm. Not a substitute for actually getting out, but for off season training, or days when I can't squeeze in a longer work out its great.