There are lots of ways to keep fit for hill walking, I prefer callisthenics myself rather than going to the gym. I came across this good article on the Sierra Trading Post web site which has some good fundamental exercises one could try at the gym or at home. If you need to add a little weight for some of these then consider using your backpack but it would be wise to not overload and make sure you pull all the compression straps tight to stop stuff moving around in the pack.
photo credit: The view from Hungerford Bridge via photopin (license)
What exercises to do use?