Runner; a person who runs. Used for competitors or people running, somewhat formal or used in sports contexts.
公園で多くの走者がトレーニングしていた。
Many runners were training in the park.
マラソン大会では走者同士のペース配分が重要だ。
In a marathon the pacing between runners is important.
Base runner (in baseball); a runner who is on base. Used in game commentary and tactics.
二塁に走者がいる場面でホームランが出た。
A home run came when there was a runner on second base.
投手の牽制で走者が戻ってしまった。
The runner was forced back by the pitcher's pick-off move.
走者が三塁にいるときは得点のチャンスが高い。
When there's a runner on third base, the chance of scoring is high.