I take it as imperative. "Get moving, legs!" - (while being chased)
Re the heart, it's pounding because I'm in the throes of an amorous attraction.
While there is the element of sarcasm, when the first person is the speaker there's also an element of sincerity. The speaker is aware of the inordinateness of the emotion, and she welcomes it.
As pure sarcasm, it might be said as an aside, by A to B, in reference to C, perhaps in a mocking way.