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. Do some extended hikes with a loaded pack to prepare for carrying loads. If you will be a firefighter, you should do some work with a tool like the Pulaski. There is no substitute exercise to prepare the back and upper body for prolonged work in the position demanded by such hand tools.