Drugs and performance enhancing methods hit distance running well before "the late 80's."
People have used a variety of substances throughout the history of athletics. Sure, with the professionalism of the sport and the increased availablity of effective drugs, MORE distance runners started using drugs in the 80s.
But Viren was widely rumored to be blood doping (in the 70s when it was legal), and I'm sure he wasn't the only one who was doing it.
Also, you can look at Marty Vaino in the 1984 Olympics. He finished second in the 10,000m, but lost his silver after he tested positive for steroids. According to Robert Voy (chief medical officer for the USOC back then, I believe), Marty insisted that he couldn't have tested positive for steroids. He was so insistent because he had cycled off the steroids months before - plenty of time to clear his system. However, what he didn't take into account was that when he also blood doped, the blood he put back in still had the traces of steroids left in it. So, in effect, he was the first athlete to be caught for both blood doping and steroid use. Read "Drugs, Sport & Politics" by Robert Voy for a better version of this story and a good insider's understanding of what was going on in sports/drugs.
And I like how Dr. Rosa is apparently the only doctor who could get his hands on EPO. I'm sure he was a big influence on both drug use and better training, but he wasn't (and isn't) the only one.
BDG