I’m sure your trip is over by now, but for future ones this is what I would suggest to get in shape for a backcountry trip:
-Try to carry your backpack as many hours during the day as you can.
-Add stair climbing into your everyday routine or workouts.
-Go for long walks and/or runs on hills, dirt road and sand.
-Lower body strength and endurance training is a must.
-Engage in a cross training exercise program that emphasizes overall physical conditioning, such as interval training, aerobic classes (involving running, spinning, swimming, etc.).
-Pay attention to your nutrition and stay hydrated.
-Wear the same shoes/boots that you would be using in your trip as many hours per day as you can even when you are working out if possible.