I'd go with something like this...

My career goal is <ideally you should align your career goal to this job that you are applying>. I'd like to see myself there in 5 years time.

To ensure I reach my goal, I'm keen to take on this role because I believe it provides me the platform to get there.
<Describe what you intend to do to ensure you will reach the goal>
<Describe what you did in the past that showed your commitment in reaching that goal>

But I'm know that I may or may not reach that goal. In short, ideally I would like to be where I want to be <career goal> but if I'm not, in 5 years time, I'm sure I would have built up my skills and knowledge in this area which allows me to be ready to take on any opportunities that come along.