Complex sentence: Maxwell never intended to become a marathon runner, and lacked the speed necessary to be competitive at shorter distances, but he persevered, and by 1977 was ranked third among all marathoners in the world by "Track and Field News".
Split into simpler sentences: Maxwell never intended to become a marathon runner and lacked the speed necessary to be competitive at shorter distances. Despite this, he persevered, and by 1977 he was ranked third among all marathoners in the world by "Track and Field News".