At 7 p.m. on October 20,1968, a few thousand spectator remained in the Mexico City Olympic Stadium. It was cool and dark. The last of the marathon runners, each exhausted, were being carried off to first aid stations. More than an hour earlier, Mamo Wolde of Ethiopia-looking as fresh as when he started the race-crossed the finish line, the winner of the 26-mile, 385-yard event.