What’s your track record in running - when you were at school what was your best distance? Also if you don’t mind me asking - roughly how old are you?
I’m afraid that typically you don’t tend to build up stamina quickly. And the best way is to go is gradually and in small increments. The rule I was told was don’t try to go more than 10% more than your last run, but if you can fit in 3 runs a week that builds up quickly. I would recommend 2 shorter runs and a long run, so if you are currently at 6k then I’d do 2x 3-4k runs at very moderate pace, and then try a long run of about 7k. Adding 10% then to each run for the 2 weeks after should get you close enough.
The real trick and one that I learned the hard way is to run much slower than you think when you’re training. One it avoids injury but more importantly it allows you to recover faster and to fit in more sessions. I’ve watched the local running group and it’s full of people trying to run faster and faster at each session, however if you watch the smart ones they run dead slow and steady, but come race day it’s these ones that fly.
Also if you’re not already doing it then I’d recommend dynamic stretching before your run, a proper warm up, and then after your run that’s the time for classic static stretches. Day after work on your flexibility and if you have one then foam rolling your quads, glutes, hams and calves.
Let us know how you get on - good luck!.