A couple of years before Miguel Indurain won the tour de france he had been sick. When he recovered he trained smarter than anyone else in the history of the sport - noone else up until then had any idea.
He recruited a team that would ride for him until they dropped (dead if necessary). They were paid 50 million dollars each for their efforts.
After he won his first tour de france it was discovered that he had a lung capacity 10 times greater than any other cyclist in the peloton, his heart was bigger than pharlaps (pharlap was a horse), and it only beat twice an hour while resting. His VO2 max was 250.
His blood lactate levels never rose above 2 mmolL-1 even when sprinting.
He was tested for drugs 5 times a day and twice each night in and out of competition and was never positive except for a couple of times but he had a great excuse.
It has also been claimed he revolutionised riding. But noone has actually said how.
So in answer to your question, I have no idea. He may have been on drugs but he does, in reality, have an exceptional phsiology.
If you want some truth about him it is out there:
and some further discussion: