Get in the best shape you can in the time you have.
Walking, jogging, biking, hiking, ect.
Know the area you hunt like it's your backyard.
Don't overload yourself with weight and pack it tight so it's not a loose awkward pile of meat on your back.
Fighting a poorly backed load will wear you out as fast as the terrain.
Make more trips not heavier ones, sounds like there will 3 people in your party....share the load, work "smarter" not "harder" run a relay system between the three of you in a rotation to get the meat to the road. That way the meat is always being looked after and everyone is able to rest while still getting the job done in a fairly nonstop manner.
Consider packing down hill to a lower road if at all feasible. In any case take the path of least resistance use your brain to lessen the pain

The relay rotation works great, It may seem to take a little longer but it is pretty much a sustainable nonstop action and if you may be packing multiple animals during your trip you wont be nearly as beaten down from it