I like a low impact workout so I use the elliptical machine with high resistance at a pretty good clip for about 90 minutes each day - good for the heart/endurance and great for the legs (thighs and calves.)  I think its also important to build up the stomach muscles to protect your back so I also do plenty of crunch type exercises with weight as well.  I find that the older I get the more economical I have to be out there.  I really agree with Snow Dog that loading and unloading heavy packs is much more taxing than the actual portage.  Getting packs on and off and hoisting the canoe fluidly really makes the difference in how far and how long you can go.