The big difference among one doped athlete and another (of same level) not using any help, is in the recovery.
The Soviet system (or the system of East Germany) was able to allow athletes to train with 3-4 quality sessions per week, and this is not possible without doping.
But this happened because, years ago, there were "myths" about how much you can earn in training, and how much you can lose if you don't do, completely out of the reality.
For example, in Lydiard's period somebody decided that one day without running could cost a decrease of the aerobic capacity. This is absolutely not true. Now we look with great interest in the combination hard work - recovery, in order to use the overcompensation, that is the real key of any improvement, in any event, and in any period.
The fact is the great methodologists had their researches looking essentially at the development of strength, in all its expressions. For that reasons Matveyev and Zachorskiy started to speak about "periodization", and Bondarchuk asked to create the "Winter Championships" for throwers, in order to have frequent modulations between basic trainings and technical training, having the goal of a final technical performance.
But this idea is not correct when we look at endurance. In this type of events, technique has little space, apart at the beginning of the activity for every athlete. The higher percentage of improvement comes from the Aerobic improvement, specifically from the increase of Aerobic Power. This depends on physiological changes training can produce in the body of the athletes, and, for doing a training able to stimulate both extension and intensity, we need FIRST to change the mind of the athletes, making normal (in their mind) level of training before considered out of the personal possibilities.
So, coming back to the original argument, doping allowed athletes to have high intensity training with little recovery, which without doping is not possible to do.
But the real question is : in athletics, to use very frequent training sessions of high intensity is the best and only way for producing top results, or is it possible to reach the same results (may be also better) in different way ? We don't have, such as in cyclism, competitions where recovery is a fundamental factor (Tour de France, Giro d'Italia, Vuelta de Espana). So, are we sure that to train at max intensity with high frequency is the best way for top performances ?
I doubt it is. For example, I alternate periods with more hard training (in any case based on a high volume of aerobic training at middle-high intensity) with periods with more "basic" training, for trying to have right balance during one season between aerobic power (which is the real key of endurance) and higher intensity, going only sometimes in the lactic area, for short periods (may be 2 sessions) before going back to a massive aerobic training in order to recruit nervous and physical energies.
It's of sure possible to have very tough training without any doping, and to reach top results in this way.
Many continue to speak about the fact African use doping.
This is absolutely not true : some of them used, but are ALL weak athletes (especially MENTALLY weak), cheated by some criminal doctor confident in their poor education and their need of finding money for surviving.
But you don't think that, if many of the best sprinters (from US and Jamaica) were positive, a lot of Russian, a lot of Maroccan, some Irish, a lot of Turky, but NEVER some Ethiopian or Kenyan (Kisorio apart), is because they are clean, and in any case able to stay at the top of the world (also beating the EVENTUALLY doped from their same Country) only thanks their talent and their training ?
There are many studies about effect of EPO : one of them was Yesterday on this website.
I want to explain that ALL THESE STUDIES HAVE NOTHING OF SCIENTIFIC when we look at top athletes. Everybody knows EPO can help the transportation of Oxygen. This transportation of Oxygen can help athletes in better training, and training is what can change our engine.
But every study is absolutely generical when the researchers speak about "training".
In the case of Yesterday, there were 4 groups : one without EPO and training, one without EPO with training, one with EPO without training, one with EPO and training.
What does training mean ? Running 30' easy is training for somebody sedentary till the previous day, is like sleeping for a real athlete. Which level of training there is in these researches ? Which level of athletes is under control ? I remember that, some year ago, there was a research about Marathon runners, considering in the Group of High Level athletes running between 2:20 and 2:50.
If this can be statistical valid in a Marathon like NY (if you want to consider "athletes" all the participants, also who finishes in 7 hours), because this involves the top 300 athletes in a total number of 45,000 (so is the 0,66% of runners), under a physiological point of view is a complete BS, when in athletics we consider two different levels of athletes a difference of 2:00 (for example, 2:05 with 2:07).
The reality is that there are many studies on the effect of doping for normal people and/or athletes of medium level, but no studies about the effect on top athletes. And, what is worse, there are NO STUDIES ABOUT THE EFFECT OF TRAINING.
Nobody can speak about the effects of doping, without knowing the REAL effects of a top training plan on the best athletes. It's time to study what we go to change with modern training for endurance in the physiology of the athletes. But, of course, this level of training (both as volume and quality, especially THE VOLUME OF QUALITY) can be applied to top athletes only, athletes of top quality, but also with organization of their life looking essentially at the best athletics results.
At the end, we can synthesize the key of training without doping (for endurance events) in this simple rule :
MORE EXTENSION IN QUALITY
NEVER TO LOSE WHAT YOU ALREADY HAVE