When you can cover the course in less that 45 minutes, add a pack with about 25-pounds. Increase the weight until you can hike 3 miles in 45 minutes while carrying 45 pounds. Also, hike hills with a pack to build leg strength and endurance. Jog the flat course without a pack to build aerobic fitness. Do overdistance for stamina, and cross-train with mountain biking and weights to build endurance and strength.
Work hardening is the third.
Work hardening is a gradual progression of work-specific activities designed to bring you to the job ready to deliver a good day's work. While fitness training provides the foundation for work capacity it is not substitute for job-specific work hardening. Work hardening ensures that the body structures used on the job are tough and ready to go. Feet are work-hardened when you hike and work in the boots you'll use in the field. Hike up and down hills and sidehills at the pace you'll use on the job.