This is the 10726th most frequent Russian word.
восьмичасовой
Eight-hour.
The word 'восьмичасовой' describes the duration of the concert (eight hours).
Мы собираемся на восьмичасовой концерт.
We are going to an eight-hour concert.
Here, 'восьмичасовая' is used as an adjective describing the length of the meeting (eight hours).
Вчера у нас была восьмичасовая встреча.
Yesterday we had an eight-hour meeting.
The term 'восьмичасовой' is qualifying 'марафон', indicating the marathon's duration of eight hours.