。
She is a runner.
Literal
She [topic-は] runner is.
Brief identification statement. 走者 ('runner') is built from 走 ('run') + 者 ('person') in a Sino-Japanese compound. The ~者 suffix is highly productive for forming 'a person who does X' nouns: 学者 ('scholar'), 著者 ('author'), 編集者 ('editor'), 経営者 ('manager'). For sports specifically, ランナー (the katakana loanword from 'runner') is more common in modern usage; 走者 is more formal/written.