If I recall correctly, Lasse Viren got gold in the 5000 and the 10000 and then entered the marathon something like two days after the 10K? He had never even competed in the marathon at all.
When a reporter asked him what his strategy was, given that he had never run a marathon before and had no idea how to do it, was that he would run with Frank Shorter. He said something like 'i might not know how to run a marathon, but Frank does!'
He then did exactly what he said, and ran right with Frank until he ran out of gas in the last mile or so, finishing fifth.
Also, and somebody will correct me if I am wrong, but I think he he won gold in the 5K and the 10K in three consecutive Olympics. I don't think he ever tried the marathon again.