The functional and metabolic processes of the endurance horse are discussed, based on personal experience and the scientific literature. Research on the energetic and physiological aspects of prolonged effort is reviewed, as regards the performance structure, described by Neumann, for human physiology.Experimental data from our previous surveys on endurance horses, competing at different levels, are also presented and discussed.The results on amino acid levels in blood, and related metabolic pathways, during endurance events, lead to speculation on the effects of the race distance on metabolic processes during long-lasting low-intensity exercise. In fact, the event distance has a significant influence on amino acid mobilization and their use as energy sources. In human athletes, the disappearance of some amino acids from blood circulation is linked to the onset of central fatigue. The effect could be similar in endurance horses; therefore, the correct intake of amino acids, and a proper feeding strategy, could improve performance.Data are also provided on dehydration and oxidative stress associated with endurance work.