Well, you need to increase the intensity of your workout. Your skeletal, muscular, and cardiovascular systems won't invest in building upon their pre-existing conditions, unless you start increasing the demands on your body. Your muscles improve by tearing and repairing and your bones improve by stresses that signal ossification. Your body is like a factory: sooner or later, the workers get more efficient at their job, but they could do more if you gave them more work; it's the same for your body - they work overtime to get more work/you give them more hours, which is like adding weight to increase intensity. You haven't really laid out your diet, but you should be following RDA values for your carbs, fats, and protein, and you should work muscle groups in a way so that they don't overtrain. If you get enough carbs in your diet, your body won't use gluconeogenesis, which is when your body uses amino acids and deaminates for energy, which is bad since you need proteins for other sources, like structural materials for your muscles. Ex. You work biceps and back, then chest and triceps, then legs and shoulders. Notice how none of those muscle groups don't really depend on each other, so you can get a full workout without overtraining and providing your muscles with rest. As you gain more muscular endurance, you can double up on muscle groups during the week.