The speaker is alone and strangely content about it. "Out of shape" could mean physically unfit, but the phrase "pleasant rot" suggests something wider: a willing slide into idleness, softness, maybe aging. Rather than feeling guilty, the speaker shrugs and says, "I will not be fit." It reads as a small, stubborn refusal to improve.
The second stanza shifts to sound. The heartbeat is compared to "talk from upstairs," the muffled noise of neighbors heard through a ceiling. That makes the heart feel both close and slightly removed, as if the body is a house and the speaker is only half listening to what goes on in it. "Rough music" is a nice phrase, since it suggests something unpolished and a little clumsy, yet still music.
The mood is quiet, wry, and relaxed, with a hint of mortality underneath. The speaker accepts decay, but the heart keeps up its rowdy presence, a reminder that life is still going on overhead.