。
She was walking along the garden path.
Literal
She [topic-は] garden [possessive-の] path [object-を] was-walking.
小道 ('small path / trail') adds charm to the image — it's not a road but a narrow garden walkway. を with movement verbs marks the route or space traversed (departure を). ~ていた captures the progressive aspect: she was in the middle of walking when observed.