For Kenyans, the altitude is fundamental. But, in my ideas, for marathon is not so important. When I was the Italian responsible, we used altitude till 40-50 days before the race in some case,till 3 weeks before in other cases. Altitude has a DIRECT effect in increasing the AEROBIC POWER, but in marathon this is not so important : our final goal is TO REDUCE THE CONSUMPTION OF FUEL AT THE MARATHON SPEED. So, altitude is very good till the period we use for increasing the Threshold (finishing about 50 days before the race), not for the last period.
But if you bring a Kenyan at sea level, his mind becomes weak, as he thinks to lose a lot of advantage.
The problem of Kenyans, in any case, is not the altitude, but the kind of courses we use. In Iten, for example, never is possible running on flat courses. Everything is up and down, and this situation, very good far from the marathon, becomes bad approaching the race, if we don't find some solution for running fast using almost flat courses. This means that, normally, the athletes hasto move for 80-100km for going to the only courses more flat that there are near Eldoret (Ziwa, Soy, Nangili etc.).
In the specific case of Boston, with Selina and Daniel we could stay in Iten or Kaptagat (Selina trains there), because we NEEDED this type of courses. The same happened in 2006 with Rita Jeptoo, preparing all the time in Iten.
So, the secret is to prepare in specific way a specific marathon. Rotterdam is not Boston. Osaka in August is not Berlin. If you think we use 4-5 months for preparing a marathon, you understand that the preparation must be SPECIFIC, because every marathon has different situation and the athletes must be ready for facing different kinds of problems.
In any case, the best Kenyan athletes are able to have a very high feeling with their "tank". Because here the control is difficult, since many training have to follow the sensation of the athletes, we need to teach them the relation between level of fatigue, type of training and effects.
Personally, I think that, if a top Kenyan could maintain his mental qualities (aggressivity, ability in suffering, high level of fatigue)training with more control in Europe, the WR could be under 2:02 in short time.
But if we move for one year a Kenyan to Europe, he cant be concentrated 100% for so long time. So, our training is a compromise between what is technically the best way, and what is possible REALLY to do.