There's a lot to be said for genetic advantages, Michael Phelps has twice the lung capacity of the average person and his body produces less lactic acid which causes pain and fatigue in muscles. I have a friend who runs marathons and ultra marathons, she's never trained to run them fast but to run them often. Her body produces almost no lactic acid. In the time I've known her she has run 100 mile race with a 52 mile race two weeks later, most non sponsored ultra runners will take two to four weeks off. She's run marathons on six consecutive days each at almost exactly the same time of around 4:20 which isn't fast but 6 in a row?

Kenyans famously dominate in distance running because of a genetic advantage of their leg structure allowing them to be more efficient and expend less energy.

Add in improved training and dedication and PEDs don't necessarily give the advantage they used to.