When oxygen delivery becomes insufficient to support a runner's aerobic metabolism, cells switch to an emergency mode in which '?

ATP is inefficiently produced and lactic acid is generated as a by-product

Glucose is the only fuel that can be used during glycolysis or breakdown of glucose. This breakdown creates ATP as glucose is converted into two molecules of pyruvate. Hydrogen is also produced during this process and if oxygen is present, the aerobic system can use hydrogen and pyruvate to produce more ATP for energy. 
However, when aerobic system could no longer compensate with the number of hydrogen being produced, the hydrogen will combine instead with pyruvate to form lactic acid. Lactic acid will then enter the bloodstream and is cleared by the liver.