。
She's not used to driving for long periods.
Literal
She [topic-は] long-time [possessive-の] driving [to-には] used-to not.
~には慣れていない is the negative of the ~に慣れている pattern. に marks what one is (not) used to, and the は adds a contrastive nuance: 'as for long-distance driving (specifically), she's not accustomed.' 長時間の運転 treats 'long-time driving' as a noun phrase.