The sport is really in sad shape. Distance running tests one basic thing and that one basic thing can be improved dramatically by drugs (unlike say football, soccer, basketball, or baseball, where even if drugs help you, you still have to do a lot of other things well)
There is simply no way that you can say anyone is clean. And the most suspicious will always be the best. The best, by definition, perform differently than everyone else. What Peter Snell, Jim Ryun did looked ridiculous at the time. We know they were not on drugs because the right drugs were not available and/or not widely in use. However, the Peter Snells and Jim Ryuns (and Genzebe Dibabas) of today are all quite rightly under suspicion. Some probably are on drugs. Some are not.
Had drugs and this message board been around for decades, we all would have questioned whether drugs were behind Roger Bannister's world record on part-time training, the rise of a random group of New Zealanders with the same coach into world-beaters, Billy Mills' kick, Bill Rodgers' dramatic improvement, and Joan Benoit's rapid recovery from knee surgery. In short, "where your dreams become reality" has been replaced by "where your dreams become reality and then you are accused of doping."