But at my back I always hear Time's wingèd chariot hurrying near (lines 21-22)
The speaker is downright paranoid about time. He sees time as a stalker out to get him. Why? Because he associates time with mortality. Perhaps death borrowed time’s chariot in order to harass the speaker. Or, perhaps time and death are one in the same for our man.
Nor, in thy marble vault, shall sound My echoing song (line 26)
Is he trying to say that, if the mistress does have sex with him before she dies, she will be able to hear his love song in the grave? It’s possible. The speaker’s vision of the afterlife for people who do have sex might be very different from the one that he imagines for people who don’t. It’s possible that he truly believes that sex frees not just the body, but the soul, from eternal nothingness.
Now therefore, while the youthful hue Sits on thy skin like morning dew (lines 33-34)
In contrast to all the death talk that precedes them, these lines are fresh and vibrant. In one of the poem’s few similes, the speaker brings the mistress back from the "grave," if you will, and tries to get her in the mood to "sport" and romp with him.