Yesterday you wrote for 15 minutes.
Today it’s going to be 30 minutes. Take yesterday’s work and change it in someway.
It could be from a different point of view, tense or even a different genre.
At the end, check on how you did.
Did having something already written to work with make it easier for you to begin and continue writing?
Did it make it harder? If so, why?
Did having to think more about changing things, hold you back?