To anybody that wonders why people would question Rupp, Farah, and others who run fast times, consider this angle: I started running, and following track, in the mid-80's. The men's WRs at the time for 5000m and 10000m were 13:00.41 and 27:13 if I remember correctly. I expected that sooner or later somebody would break the 13-minute and 27-minute barriers. What I didn't expect was that within a few years, everybody and his brother would be doing it. In a short space of time, sub-13 became something that relatively anonymous runners were doing with regularity. In retrospect, it is clear that this surge in distance running performance coincided with the rise of EPO. This doesn't necessarily mean that "everybody was doing it", but you would have to be hopelessly naive to believe that some of the improvement wasn't due to EPO.
Gradually, drug testing seems to have gotten better, or so I am told. There is a test for EPO, although it is hardly foolproof. Drug testing comes down to the details - how often are athletes tested? etc. Anyway, times for the 5000/10000 have basically stalled at where they were in the late-90's. They definitely haven't gotten significantly worse. Basically, people are running about the same times in this supposedly "cleaner era" (the opposite of what has happened in cycling, by the way). So what gives? Did the PED's not help them in the first place? Maybe, but I don't believe that for a second. It's not like there weren't amazingly talented runners willing to train like madmen prior to 1993 or whenever.
Bottom line: when EPO came out, endurance running to markedly faster. Times are roughly the same now, so why wouldn't we at least question Rupp/Farah and the like, when they are a good bit faster than Said Auoita, Henry Rono, etc? The training is still the same - intervals, strength work, etc. The standard excuse "we train smarter" sounds like a pretty standard soundbite from dopers of the past: "I've learned to rest more" or "I've really been working on my nutrition and getting lots of rest" and so on to explain huge improvements. As if nobody had thought of those things before.