how to get in shape for backpacking?

[Troy]

I have a camp coming up where I raft 50-100 miles in class 4 rapids. Then we hike and backpack. I bench 155, squat 250, weigh 130, can run about 5 miles, and run a sub 5 minute mile. I am not worried about the rafting but I need to get into really good shape for the backpacking. Any suggestions