Not unattractive but getting on.
to get on means: get along to have a good relationship.1
Does it mean:
she is attractive but she doesn't set along to have a good relationship?
The context of the phrase is:
The writer herself is rather austere looking. Dark hair pulled back from her face in a tight ponytail. Strong bones. No-nonsense skirt and sweater, equally no-nonsense eyeglasses. She looks like she might make a competent nurse. The only flourish is a pretty scarf around her neck. Not unattractive but getting on. Maybe pushing forty. Lauren wonders idly about the book she’s writing.