Viren ran 13:55 already at age 20, 3 years before the 72 Olympics. Plenty of time to improve, doping or no doping. He achieved that PR with only an annual volume of 2,927km. Plenty of room to up the mileage and intensity as well! In 3 years he cut 29s off.
"The story I've heard is that the guy never did anything except in an Olympic year"
The story you heard. That says it all.
I think Viren's success factors were:
- Amazing consistency. Rolf Haikkola planned Virens training extremely well. Viren didn't take shortcuts. He ran every training session. With some hard sessions Haikkola was there to make sure the workout is as lung searing as it's supposed to be. Haikkola's tongue-in-cheek nick name in the running circles was "raaka-Rolle", or "brutal Rolle" (Rolle comes from Rolf).
- Right from the beginning Haikkola and Viren agreed there is only one goal. No compromises.
- Viren's life was focused on running. He was free to go on training camps all over the world despite his job. He also had a small athlete's scholarship.
- Cooperation of Haikkola and Viren in developing the race strategies. They weren't afraid of trying new things in races of lesser important (other than Olympics, that is). In the Olympics Viren was ready. He knew exactly what he was capable of. He also knew pretty well his competition.
- More intense training than others' who did big mileage. It was just within limits so Viren stayed relatively healthy all the time.