Complex sentence: Starting in the late 1960s, the IHL's quality of play significantly upgraded until by the mid- 1970s, it was on par with the American Hockey League (AHL), the longtime top feeder league for the National Hockey League.
Split into simpler sentences: Starting in the late 1960s, the IHL's quality of play significantly improved. By the mid- 1970s it was on par with the American Hockey League (AHL), the longtime top feeder league for the National Hockey League.