On Tuesdays and Thursdays I hike on the treadmill (I live in the city so I usually run outside but hike on the treadmill because the city is so artificially flat). By hike I mean I set the treadmill at a strenuous walking pace between 2.5 and 3.5 and a high incline. A half hour of this and my calves try to tear through my skin. In practice I march with between 50 and 125lbs of kit. I want to be able to simulate this by packing my hike bag to only 40 or 50lb next time I get on the treadmill (and a slower pace). But how do I do that? I can’t bring guns, missiles and bullets to the gym! I assume I can put weights in the bag, but my free weights shift a lot in the bag. Is there a tried and true way to weight-up a bag for hiking? Are there special weights available? Should I be using a particular type of weight (ex hex, dumbbell, wheels)?

-I will use the same bag I hike my kit in to minimize back strain. My goal is to get more accustomed to the hike and to build a little more shoulder and calve muscle.-

For clarification- I mean at marching speed, not running or jogging speed.