Aiming for a set time for a set distance, how do i best achieve it?

I need to get to 2.5km in 12 mins. The other day I ran 3.7km in 21.5 mins. I only have a month to achieve my goal.

Do I keep running longer distances and hope speed comes with that, or should I just run the 2.5km as fast as I can in place of my normal running?