I'm sure you've done this, but put your ruck on your back and march - make it as heavy as you can and still maintain a very quick pace -almost to the point where you start breathing really hard. It will not only help with your stamina (assuming you push yourself), but you'll also build overall strength (especially in your legs and back).

If you're tired of ruckmarching, you can try weights in some sort of circuit training routine. For example,
10 Burpees, 10 Power Cleans, 10 Military Press, 10 squats, 10 deadlifts, 10 squat jumps. Repeat for 20 minutes with minimal rest. Basically any kind of hard work where you limit your bodies ability to rest will increase endurance and stamina. Running hills/sand is my favorite and I'm sure there is enough sand around...Stay safe.