If you feel stuck at running at a particular speed and would like to increase your pace, I highly recommend interval training. Usually when I am in the gym on the treadmill I use that indoor exercise for interval training. My instructor has set up a schedule that takes me through different speeds between 8.5km/hr to 14km/hr in 1 to 2 minute intervals.
Some of it is very intense and after running at 14k falling back to 8.5km feels like advancing at snail’s pace. But after doing this for a few times, it’s easier to increase your speed during regular runs.
When going through the intervals, I always have to suppress my tendency to increase the speed when I am running at a slower pace, but I know that the differences are beneficial in the long term.
The treadmill in my gym can be programmed to run my personal program from a USB stick. If you have something similar in your gym, that makes interval training very convenient. If not, you can simulate by switching speed every 2 minutes. One thing to keep in mind is that you have to alternate different speeds to benefit the most from this approach – so as example, go from 8.5km to 11km to 9km to 12km to 10km.
If you really want to spice it up you can also alternate running at different speeds with walking. So run at a high pace for 2 minutes and alternate with 1 minute at walking speed (4.5-5km/hr depending on your pace).