It is usually runners, since you are referring to four separate runners, and the verb (are) is plural:
The first, second, fifth, and eighth runners are wearing blue.
One way runner could properly be singular is if you put it after every adjective:
The first runner, second runner, fifth runner, and eighth runner are all wearing blue.
As English sanctions elimination of repeated (and redundant) words, the following is also grammatically correct:
The first, second, fifth, and eighth runner are all wearing blue.
However, this is much less common, since most people instinctively do not prefer to put a singular noun (runner) directly before a plural verb (are).