。
She stood waiting for me for a long time.
Literal
She [topic-は] long time standing me [object-を] was-waiting.
長い間立って待っていた stacks two actions: 立って ('standing') as the manner, and 待っていた ('was waiting') as the main progressive verb. The image is of someone physically standing in one spot, patiently or impatiently waiting. 長い間 ('for a long time') sets the duration.