Further to Beth's answer, using arduous also sometimes suggests that the task etc. is one that is boring or that you'd rather not have to do. "I began the arduous task of sorting out the good apples from the bad ones."

Arduous is not the same as difficult. A task that is arduous is a task that requires a lot of effort and is tiring, it may or not be difficult. For example walking is not difficult, but walking for 6 hours without a break would be arduous.