Mitochondrial disease - why can people who have this, only exercise for only short periods of time?

Energy deficiency: Mitochondria are the batteries of the cell, with about 1000 per cell. They transform chemicals using oxygen (oxidative phosphorylation) to produce high energy phosphates (atp, etc.) that power muscles, heart, and brain. Mt disorders cause eye muscle deviations, low muscle tone, muscle cramping, decreased cardiac performance, and, in severe cases, intellectual disability.

Cellular metabolism: Mitochondria are the energy "factories" which produce energy and handle metabolism of fuel sources (oxygen and glucose) in our body's cells. Mitochondrial dysfunction can lead to impaired cellular mtolim and premature cellular dysfunction and injury during exercise (cells can't keep up with the embolic needs during exercise).