But cyclists, if want to win something, needs in any case the ability to go, for more long time, at high level of lactate, and this is something we don't need for Marathon runners.
Of course if we speak about a "fuga" (when some athletes escape in a small Group, and they have to push hard for many minutes) that part of race is similar as a Marathon effort. But it's a fact that in cyclism there is an alternance of efforts, also at even speed, depending on many other factors (for example, at the same speed, when you go to lead the Group you can go at 180/190 HR, and after you stay for 4'/5' in the Group at the same speed, and you can lower your HR at 150).
During a congress about high altitude training, I was one of the speakers, another was the Italian Head Coach of our Olympic cyclists, Antonio Fusi, who won several medals with the competition of 100 km (tempo) for teams (4 cyclists per team).
He prepared in altitude the "chronomen" in Livigno (1800m), because the perfection in changing the positions was one of the secrets of a good performance. The cyslists, at a speed faster than 50 km/h, had to stay in front for 30", and after had to learn how to go at the end of the team in right way. This means everybody had 30" of max effort in front, and 1'30" at the same speed, behind another cyclist.
He showed the HR of these cyclists, who in 1'30" at the same speed were able to reduce their HR from 190 to 165/170, only for the big difference they had to face about the resistance of the air.
With the increase of the speed, we have to change completely every parameter, and these two sports are not really comparable.
At least, another important factor is that, in cyclism, EPO was only the base for the real doping : GH and other steroids, used for increasing strength and specific resistance.
If you are a runner, for increasing your speed you need to enhance the ability to transport oxygen, but especially the ability to extract it from the atmosphere. You don't need to increase your muscle strength, since everybody has a big margin between his max speed and the speed of the race (in the case of Marathon, for athletes running at 3' per km = 18" every 100m, we can easily suppose they can run 100m in 12", that is 150% of the speed of the race. When we have in cyclism an average of 45 km of speed for a race lasting 5 hours, which means 1'20" per km = 8" every 100m, if we want to apply the same ratio, we need to suppose all these athletes can be able to go at their max speed using 5"33, that is the speed of the best sprinters on track, so obviously not possible), while in cyclism the most important factor of improvement is to be able to use one tooth more, at the same frequency. This means the main doping is for increasing STRENGTH, maintaining the same endurance.
About the first part of your post, I can agree with you that, for athletes not so mentally strong, EPO can give a different self confidence pushing them to do in training what never they tried to do before. But this is exactly the reason because ALL the WR holder in long distances are clean : because nobody IF NOT MENTAL STRONG can become a WR holder, or an Olympic Champion, or a winner of a Major, since the mental strength is the most important quality for this type of event.