Im about to pen an email that will probably get me "let go"

Hi im 27f with spastic hemiplegia. I recently landed a job running meals to patients in hospital. Its physically demanding. I can manage but I feel like i need to be capped at 30 beds / one floor of the hospital. Admits can vary and callous happen so 30 beds isn't usually the case. How should I begin to approach this with my boss?