isn't the only place where you can be challenged and can grow.

If your work isn't challenging you, then provide that challenge yourself outside of work. Learn to play an instrument, learn a new language, or learn a new sport. Go become an "expert" in some field of knowledge that is new to you and has nothing to do with how you earn a living. Set some fitness goal that will stretch you, and apply yourself to reaching that goal. Whatever interests you and is hard for you, go do that until you get good at it.

Getting paid to learn something new or develop yourself is awesome if you can manage it. But if not, don't let that stop you from growing and challenging yourself.