People usually have so much to do before an expedition that they neglect their fitness. Often if you have good base level of fitness you can use the early part of the trip to get in shape. For those over 35 there is in this more extreme strategy the increased possibility of injury to a tendon or ligament usually. Muscles tone up fast - two weeks can see a return to past glories but bones, ligaments and cartiledge take three months minimum, or at least should be allowed as much.
The best fitness program concentrates on brute strength rather than mere stamina if you have to choose - why? Because injuries usually occur through the sudden overstraining of your body when hefting something heavy. But then you need stamina too if the expedition requires paddling or walking with packs or walking along river banks for hundreds of miles. The slant of a river bank can over days and weeks lead to a version of shin splints as I discovered in Canada a few years ago having walked 400 km along the north bank of the Peace River in Alberta.
Sledge towers train by towing tractor tires tied to their waists. Canoeists can train in a pool by having a bunjee tied to one end of the canoe with a scale on the pool side - they have to strain against the bunjee cord to make a certain mark. Inventiveness is key in exploration fitness acquisition because of its deadly dull (usually) nature. Climbing walls are great for upper body and fun too. Mountain biking up hills and down dales is as good as running for aerobic – or almost as good. I have found that running truly is the 'king of exercise' as the master Bruce Lee put it. Unfortunately it is bloody boring even in the beautiful wadi digla area where I train in Cairo. My new and exciting solution: take the gps and turn each run into a number fest. I constantly check my speed as I go up hill and down dale. I measure the distance I have gone and have to go. I cross check this with previous speeds and times and pretty soon I've run 2 km then 4 then 8. That's where I'm at right now. Train in the hills by carrying jerrycans of water in your rucksack - climb the hill with it in and then dump it for the trip down to save your knees; which are better built through biking and climbing and yoga than the shock impact of pounding down a hill 20kg heavier than your usual weight.
The best way to train is always with a buddy.