Normally, ὁπλιτῶν δρόμοι would mean "races of men in armour." In one instance, however, Pindar uses ὀπλῖται adjectivally to modify δρόμοι: cf. J. B. Bury, The Isthmian Odes of Pindar (London 1892) s.v. Isthmian 1.23:
ὁπλίταις occurs only here in Pindar. ὁπλίταις δρόμοις, "mailed races"; a close paraphrase of the technical ὁπλιτόδρομος. With this phrase compare ἀσπισταὶ μόχθοι, Euripides, Electra 442.
So ὀπλῖται δρόμοι → mailed races, but only once in Greek literature.