"
in, shaping up, then checking out on the prison bus
This is
"I'm breaking in shaping up then checking out on the prison bus" refers to how very quickly one is plunged into the realm of cancer, having had paediatric cancer I have bore witness to the community that revolves around children's cancer (incidentally it is an underrepresented disease which is in dire need of more attention!), it discusses how at first one feels as if they don't belong, there are always those who have it worse "breaking in", "shaping up" refers to the way in which one quickly becomes immersed in the culture and jargon of the whole affair. But all to soon circumstances change and you feel alienated yet still tethered to the cancer.