Not all mental. SOF operators maintain a high level of cardio and strength performance by working both very hard. They are typically stronger pound-for-pound than a gym-rat who focuses on muscle mass, but slower than a pure-endurance athlete who doesn't carry any extra weight. It's a constant tug-of-war between strength and cardio, slack off on one and you'll know it fast. You must have a natural endurance ability as a basis for that sort of regimen..anybody can build strength with diet and weights.