The hiring manager is trying to find out if you’re prepared for the stresses, commitment and expectations on the horizon. Do not attempt to minimalise the upcoming challenges as you’ll come across as under prepared for dealing with adversary.
Analyse the job specification and highlight an area which seems ambitious or daunting and then welcome the challenge to conquer it.
A good answer could be:
The job description says I’ll be responsible for line managing a team of twenty staff; line management isn’t a responsibility I’ve had before so it may be challenging initially. However I feel it’ll become second-nature after I learn the company’s Code of Conduct and disciplinary procedure. Also I’m looking forward to getting involved in the recruitment process to identify new talent and then support their development and training.