Green’s Dictionary of Slang

(US) to be a veteran, to be experienced; esp. used (derog.) of a woman.

1971		Winick & Kinsie Lively Commerce 43: The old whore is a ‘hard leg’—she’s got a million miles on her.
1994		P. Blauner Casino Moon 195: ‘Well, at least Anthony went for a woman with some miles on her, and not some bimbo like I thought he would’.