An athlete who would like to optimize endurance performance would do well eating what type of diet? what is the metabolic pathway being used and why would this diet benefit them?

The main group of macronutrients that athletes need is carbohydrates. That's because our bodies transform them quickly in simple sugars (glucose). Part of that glucose will be for maintaining the normal body functions and the rest will be transformed by our liver into glycogen (Glycogenesis). 
Later, when exercising, this glycogen will be converted into energy and that process is called: Glycogenolysis.
 Unlike what people think, protein is not the best source of energy because it will only become energy if the body ran out of carbohydrates or fats.
For a optimized performance, nearly 65% of the athlete's diet should consist in carbohydrates. 
This diet will benefit them, also after the exercise because it will give less muscle pain.