You don't have to write a novel each time you sit down to write, either at your desk or your computer. It is enough that you have actually WRITTEN something (well as long as they make vague sense and convey something to the reader - even if it is only yourself!!).
It is important to start the HABIT of writing, and you have to start somewhere and sometime, so start TODAY - now after you have read this prompt - no time like the present eh?
Define yourself in say - five sentences. Just freewrite for a few moments, in words, notes, just let your mind and pen wander. Then pick out the pertinent bits of how you would define yourself.
For example, I don't define myself through other's eyes and standards, I define myself by what I have inside and what I want to do and be in the world.
So I would go off on a five sentence rant about how I want to change the world one day at a time and where I would position myself to be most effective at achieving that change - today that would focus on something specific tomorrow it would be something else...
Some words will jump out of this free writing as most important, so work on them for a few more minutes.
There you go... writing done... and if you find this too personal, write it through the eyes of a fictional character... !! sometimes you never know which is which - am I fiction or am I real ? who cares...?