He was named Dutch Sportsman of the Year in 1975. He was a 10,000 metres finalist at the 1976 Summer Olympics and set the world record for the hour run the same year. Hermens twice improved the world hour record, on the Papendal track.
In September 1975, with Gerard Tebroke as a pacemaker, he ran 20,907 meters.
In May 1976, without a pacemaker, he ran an additional 37 meters. This record held until 1991.
After his athletic career was cut short by injuries, Hermens worked for Nike, leaving it in 1985 to start his management company, Global Sports Communications. Hermens" company manages in excess of 100 athletes.
His current and former clients include Haile Gebrselassie, Kenenisa Bekele, Gabriela Szabo, Nils Schumann and Hezekiél Sepeng.
Hermens withdrew from the 1972 Olympic Games following the Munich massacre. "lieutenant"s quite simple," he said. "We were invited to a party, and if someone comes to the party and shoots people, how can you stay?".