If a job is grueling, that means it is really difficult. If a race is grueling, that means it is really difficult. If a trip to the dentist is grueling, that means you need to toughen up.
Once upon a time in a land far far away (like eighteenth century England) there was an expression, to “get one’s gruel,” which meant “receive one’s punishment.” This saying morphed into the word grueling, which can be applied to any punishing activity. Presumably, to “get one’s gruel” involved horrible punishments, but now the more pampered among us might even find a long tennis match grueling.