Increasing red cell count through EPO/doping will raise the aerobic threshold of an athlete by allowing more oxygen per unit time to be delivered the working muscles. Therefore the muscles will be able to work aerobically at a higher work rate as more oxygen is available through its enhanced delivery to the muscles. When working aerobically the muscles will be able to use more fats/triglycerol as fuel rather than glycogen only. These substrates yield more energy (ATP) per unit mass than glycogen. Therefore over time the muscles will be less gylcogen depleted.
Yes, I understand how increasing the concentration of red cells in the blood would allow an athlete to run faster aerobically. But does that really result in a shift from glycogen (glucose) to fats as metabolic fuel? Can you cite any evidence for that claim?