Endurance runners use blood doping... That's how they 'cheat'.. Puts extra stress on your heart though..
ANd blood doping does what? Increases RBC, more RBC more oxygen. At a point however the blood becomes too thick that the heart has to work harder so the benefit of extra RBC is no longer beneficial.
Guess what many AAS do? They increase RBC and in turn potentially can benefit oxygen carrying potential. The problem is, with endurance training the narrow of margin where each individual can benefit is small and takes patience, trial and error and a lot of blood work to determine your specific ideal RBC value.
That being said, athletes are well know to suffer from magnesium deficiencies. This can make a HUGE difference in the heart rate required and amount of oxygen needed to perform a given task. Get an RBC magnesium test and if your not at 6.8 mg/dL you need to take more.