The gradual increase of the difficulty of your training (progressive overload), is the only guaranteed method to increase your fitness. This works whether your goal is to get stronger, get bigger, get more endurance, etc. Your body changes because it adapts to the changing levels of stress.
Every change or shock in the environment is perceived as stress and the body tries to adapt to it. If the stress is too small, it won’t cause a disturbance and no adaptation will occur. If the stress is too big, you simply die.
