Provided you can already run or walk the course in 45 minutes, you'll want to add weight during your training. Start with 10 to 25 pounds and slowly increase the load in 10-pound increments until you're hiking at an incline with a 45-pound pack.
Keep in mind that as you add weight, it will become increasingly difficult to maintain proper posture. Focus on standing tall with your chest up and your shoulder blades drawn back and down, and avoid bending over at the waist as this could slow you down and increase your risk for injury.
Do this two to three times a week. Then, twice a week, run, bike, or hike hills at a steady pace and lift weights (prior to your cardio) to build strength and stability.-- Paul Robbins, metabolic specialist, Athletes' Performance