Swimmer Mark Spitz, then an 18-year-old high school graduate from Santa Clara, Calif., returned from the 1968 Olympics with two gold medals, one silver and one bronze—and a feeling of failure.

Mark Spitz finished last and four seconds off the pace Thursday in a five-man 100-meter butterfly race at Santa Cruz de Tenerife, Canary Islands. It was his first 100-meter race since the 1972 ...