runner has 10 distinct synonyms spread across 5 of its 10 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
someone who imports or exports without paying duties
a person who is employed to deliver messages or documents
“he sent a runner over with the contract”
a baseball player on the team at bat who is on base (or attempting to reach a base)
a horizontal branch from the base of plant that produces new plants from buds at its tips
a trained athlete who competes in foot races
(football) the player who is carrying (and trying to advance) the ball on an offensive play
Synonyms, antonyms and the broader and narrower relations for runner come from Princeton University’s WordNet. Full sources and licences.