Why is lactic acid fermentation useful for short bursts of energy but not for meeting a long-term energy demand, such as running a marathon?

Lactic Acid is Produced in muscle cells from the reduction of pyruvate (under anaerobic conditions) to regenerate NAD+ so that glycolysis can continue. A rise in lactic acid usually accompanies an increase in physical activity.

Slow twitch is the type of muscle that contracts slowly and is fatigue resistant (marathon runner)

Fast twitch are muscle fibers that react quickly and fatigue quickly (track runner)

Anaerobic respiration requires 2ATP and doesn't require oxygen

Meanwhile, aerobic respiration requires oxygen and has 36-38 ATP

ATP (adenosine triphosphate) is the main energy source that cells use for most of their work

The lactic acid fermentation is useful for the short bursts of the energy and cannot be used for meeting the long term demand as running a marathon.

This is because lactic acids gets deposited in the muscles and during the physical activity. This happens when there is not enough oxygen in the body and the ATP is produced in absence of oxygen.

Lactic acid build up in the muscle can create cramps. Lactic acidosis usually occurs because of intense exercise and can cause pain and weakness in the muscles. This occurs when there is a deposition of lactic acid in the body.

Hence, it is not suitable for long term energy demand.