"we worked during the night and during the day." Here "night" and "day" are a merism and they mean "all the time." Alternate translation: "we worked all the time" (See: [[rc://en/ta/man/translate/figs-merism]])
Paul emphasizes how hard his circumstances were. Difficult labor implies work that requires great effort. Hardship implies they endured pain and suffering. Alternate translation: "in very difficult circumstances" (See: [[rc://en/ta/man/translate/figs-doublet]])