Edwin Moses. He'd only taken up the 400m hurdles event just a few months earlier. Yet he won the '76 Olympic gold medal (in this video) in a romp, and then after losing a few times over the next year or so, he won 122 consecutive races. Incredible. And as mentioned, at the time of his first Olympic win he was attending Morehouse College-- on an academic, not athletic scholarship.
Like Juantorena, he took huge strides.