Three short lines, and a whole late-night scene. The speaker is working from a home office, and the title, "Long hours," tells us the work has stretched well past a normal day. It's dark, it's quiet, and the only company is a pen.
The most interesting choice is the verb. The speaker and the pen "eat the night up," as if the dark hours were a meal being devoured. That could suggest hunger or drive: a person absorbed in writing, using the night rather than merely enduring it. But eating also means consuming, and the phrase can hint at a cost. The night is being used up, and so, perhaps, is the speaker's rest.
Notice too that the pen is treated as a partner, not a tool. "The pen and I" makes them a team. The mood is solitary but focused, with a faint tension between satisfaction and exhaustion. The poem doesn't tell us which wins out, and that ambiguity is what makes it linger.