Whenever you eat, if your body fat is above about 3%, order a salad, as it's the only food that doesn't add fat. Eating most foods will cause CJ to gain some fat, but food is also very important to building stamina and muscle mass.
Lung capacity isn't exceptionally useful except in a few scenarios. Finding oysters on the ocean floor also has a minor effect on it, and after collecting all fifty hidden oysters, lung capacity is removed altogether and you can stay underwater for as long as you wish. Lung capacity can be trained by swimming underwater until the last second and then resurfacing for air. Body fat can be lowered to about 5% with no ill effects get rid of it completely, and CJ may lose muscle mass during strenuous physical activity.
You're aiming for a rating of around 75-85% to maximize physical strength without sacrificing speed or agility. To build muscle mass, lift weights at the gym. In addition to stamina, CJ has muscle mass and body fat whose percentages can be altered by working out.
