When oxygen-related energy is not enough to match the request, your muscles activate a second metabolic pathway, called glicolisis, to produce more energy from glucose, without oxygen. You can imagine that during a gradual incremental effort, at low intensity you produce energy throught the aerobic system, burning a mix of CHO and fats. Increasing the intensity, you reach a point where you start to produce LA, but stilll not enough to accumulate, since the speed of production is lower that the speed of metabolization. At a higher intensity, the production speed becomes predominant and LA levels in the blood start to rise. The more efficient the aerobic system becomes, the lower the need will be to start the LA mechanism.
Many factors affect the oxygen availability to the muscles.
It is not my goal to analize each of these factors. Instead , I would like to simplify by focusing on training for the central and peripheral factors. In general, even if the real picture is more complicated, the first factors are affecting what I called the aerobic power, while the second factors are influencing the aerobic capacity. Basically, power and endurance. Aerobic conditioning requires at least 2 or 3 training sessions a week, depending on your starting level and the goals you want to achieve.