Work capacity is a composite of fitness, acclimatization, nutrition, skill, experience, motivation, and intelligence. Fitness is the most important factor. Fitness has two components, aerobic and muscular. Aerobic fitness is a measure of your ability to supply working muscles with the oxygen they need to perform vigorous day-long work. When you can deliver and use oxygen efficiently, you can do more work without undue fatigue.
Muscular fitness includes strength, muscle endurance, and flexibility.
Strong workers can lift and carry heavy loads with less fatigue or risk of injury. Muscular endurance enables you to continue working at otherwise fatiguing tasks. And flexibility means a better range of motion that lowers the risk of injury. Fitness is the foundation of work capacity. That's why resource agencies stress its importance, and why you should report to the job fit, work-hardened, and ready to take on tough field assignments.