Color green: Protons transferred from NADH (HR) and from solvent (H+) in the reduction reaction.
Only a small fraction of the energy of glucose is released in the anaerobic conversion to lactate. Much more energy can be extracted by means of the citric acid cycle and the electron transport chain. The formation of lactate by the marathon runner (an aerobic organism) buys time.
Lactic acid (lactate) produced in muscles is transported to the liver where it is (re-)converted to glucose for further energy or storage needs. This shifts part of the metabolic burden of active muscle to the liver.