Complex sentence: The Marathon Stakes, formerly known as the "Breeders' Cup Marathon", is a Grade II American Thoroughbred horse race is a Set Weights with allowances race open to horses three years old and up, and contested on dirt at a distance of miles.
Split into simpler sentences: The Marathon Stakes, formerly known as the "Breeders' Cup Marathon", is a Grade II American Thoroughbred horse race. It is open to horses three years old and up, and contested on dirt at a distance of miles.