Complex sentence: Coghlan was nicknamed "The Chairman of the Boards" because of his success on indoor tracks having set the world record in the indoor mile run at 3:50.6 in 1981 then bettering it to 3:49.78 in 1983 at New Jersey's Meadowlands indoor arena.
Split into simpler sentences: Coghlan was nicknamed "The Chairman of the Boards" because of his success on indoor tracks. He lowered this to 3:50.6 in 1981 and then bettered it to 3:49.78 in 1983 at New Jersey's Meadowlands indoor arena.