The Soviets didn't field an Olympic team until 1952 yet managed to haul off a boatload of medals by 1956. Kenya got a bronze in the 800 from Wilson Kiprugat (that may not be spelled perfectly) in 1964 and then came Keino and his successive golds in 1968 and 1972 on top of the golds from Biwott and Temu in '68. And regardless of when Ethiopia got round to fielding their first Olympic team, they managed to produce Bikila and Wolde, four successive medals in the Olympic marathon, three of them gold. A fair amount of talent does seem to have developed itself "overnight."
In the 70s, there were letters to Track and Field News complaining about Kenyan runners at US colleges and there were plenty of Kenyans on the US road circuit by 1980. And US runners, some of whom trained in Boston, managed to beat them with some regularity. You might want to check out a bit of history yourself.