Complex sentence: In 1975, Keulen-Deelstra switched to marathon skating and she became Dutch Champion in that discipline fifth times; her last Dutch Marathon Championships title being in 1980 when she was 42 years old.
Split into simpler sentences: In 1975, Keulen-Deelstra switched to marathon skating and she became Dutch Champion in that discipline five times. She won her last Dutch Marathon Championships title in 1980 when she was 42 years old.