Complex sentence: At the 1952 Summer Olympics in Helsinki Zátopek won gold in the 5 km and 10 km runs, but his final medal came when he decided at the last minute to compete in the marathon for the first time in his life, and won.
Split into simpler sentences: At the 1952 Summer Olympics in Helsinki Zátopek won gold in the 5 km and 10 km runs. His final medal came when he decided at the last minute to compete in the marathon for the first time in his life, and won.