800-metre-long 用了hyphen之后，这三个单词合在一起作复合形容词修饰race,
[800米长的] 长跑（比赛）。
800-metre long race, long单独修饰race，意思成了， a long race that is 800 metres in length.
800 metre之间如果没有横线，metres要用复数，但它不能做定语修饰后面的race了,
The race is 800 metres.
类似： He is a five-year-old boy. VS He is five years old.
