This is the 2967th most frequent Korean word.
최장
"최장" means "longest" or "maximum length."
'최장' is used here as an adjective meaning 'longest' to describe the record.
그는 최장 기록을 세웠습니다.
He set the longest record.
'최장' is describing the 'distance' as the longest one in the event.
이 최장 거리 달리기는 많은 참가자를 끌었습니다.
This longest distance run attracted many participants.
'최장' modifies '시간' to denote the extent of duration as the longest.