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If I told a friend about this piece, then I would say that it is about the race against time that every person faces. You present the classic problem; no person has all the time in the world, and so we must make every moment count. As you say frequently, "Time flies, so I have to run". This represents the constant need to beat time, to accomplish everything that you want to accomplish before time runs out. All in all, this poem is about making sure that you use your time wisely, not wasting a second and therefore taking every moment to do something meaningful.
The repetition of the phrase "Time flies, so I have to run" is very effective, as is your constant rhyming scheme. Both can be used to represent either the fast paced nature of the narrator in the poem, their need to do everything they can before time runs out, or can be interpreted as the fast paced, "flying" nature of time. When reading this poem, every line flows very quickly from one line to the next without stopping due to the rhyming scheme, and this is very effective is conveying your ideas.
There are two ways I think you could improve this piece:
1) Add more stanzas. Your poem, while good, feels incomplete. While that might be intentional so to contribute to your overall ideal of the nature of time (fast paced and unrelenting, until it suddenly stops and that's it), I feel like your poem could benefit from another stanza or two to contribute more meaning to the poem and give it a more complete feel.
2) The line "try my hand at a cool bun" does not fit at all. Unless you have some particular attachment to this, I would change it. Perhaps something like "No room for rest I've only just begun" or "Zoom like a bullet fired from a gun" or something similar. I'll leave that up to you.
Nice poem, I hope some of my advice has been useful. I look forward to reading you next piece of writing :)