from what you are writing, you are focusing on distance way too much.
I'd suggest you try to focus on time running more. Also for me (and im tempted to say in general) it is better to focus on HeartRate rather than pace.
so for example a Training Week could be something like this:
Monday: Rest
Tuedsday: Run - 60Min, HR ~160 (which is roughly 75-80% of my MAX HR)
Wednesday: Rest
Thrusday: Fartlek ~50min
Friday: Run - 40min, HR ~160
Saturday: Rest
Sunday: Long Run: 90min, HR ~150-160
So by Training with time and HR, I am gradually improving distance/pace.
I am currently running 90mins at around 5:40/km (thats close to 16km) quite comfortably.
Anyways, try to practice for a given time and build endurance rather than running way too fast with a distance goal.
while running you can eat those: click