Go-Ruck and 5.11 (among others) make weighted plates that fit seamlessly into their backpack systems, allowing you to strength train without risking an unbalanced pack and upping your risk of injury.

Start with a 2 mile distance. Grab your backpack and load it with 10% of your bodyweight. For instance, if you weigh 150 pounds (68 kg), then you would load your pack with 15 pounds.