strenuous
Meaning: requiring tremendous energy or stamina
Usage: Running a marathon is quite a strenuous task. So is watching an entire Star Trek marathon.
arduous
Meaning: difficult and requiring great effort
Usage: The hikers completed the arduous climb after several hours on the trail.
laborious
Meaning: requiring hard work or effort
Usage: The students found the research project laborious, but they were proud of the final report.
taxing
Meaning: physically or mentally demanding
Usage: Preparing for three exams in one week was a taxing challenge.
grueling
Meaning: extremely tiring and demanding
Usage: After a grueling practice, the team rested quietly on the bus.