。
She ran with all her might.
Literal
She [topic-は] with-all-one's-might ran.
一生懸命 with 走る paints not just running but pushing the body to its limit. The 四字熟語 originated as 一所懸命 ('staking everything on one place') — a samurai's defense of a single piece of feudal land — and softened over centuries into the modern adverb of pure effort. The 'whole life' reading of 一生 is folk reanalysis; the original 一所 ('one place') has been almost entirely supplanted in everyday writing.