How can I train to run/jog/march while carrying 50 to 100 pounds. Such as training to do take the Fan Dance (SAS March) Fan Dance

On my first attempt to train, I putted two 10 pounds ankle weights and some heavy books in a backpack (totaling about 25 pounds), when running I had pain in the rhomboideus major area.

The next day I ran without the backpack and I felt light as feather, but the back pain worried me.

How can I increase weight, without harming my back? The backpack does not bounce and I always make sure to stretch, is there any secrete in the army to running/jogging with heavy weight?

The weather where I train is uneven, cold and snowy. I use a 5.11 Rush 24 backpack and blackhawk boots.