To get to the next level of running you can:
- Improve your technique
- Improve your conditioning
- Improve your muscle strength and neuro-muscular control
Each of these will help you improve your running economy and performance.
This article, Methods of Improving Running Economy by Coach Brett, has a nice discussion of different ways to improve performance including the following summary:
High Intensity Interval Training (HIIT)
Strength Training, Core Strength Training and Plyometric Training
Running Analysis and Biomechanics Modification
One study that he references used:
"a combination of heavy weight training (hamstring curl, leg press, parallel squat, leg extension, and heel raise) and endurance training (running) led to an improvement in running economy compared to endurance training alone in well trained triathletes."
Another study used Plyometric Training and found that:
“After 9 weeks, running economy at 18 km/hour improved in the group that performed plyometric training, but remained unchanged in the control group."
And another Core Strength Training study:
.. "a program consisting of 4 core-training sessions per week for 6 weeks led to improved 5K run performance in marathon runners"
From a biomechanical standpoint:
"faster runners tend to have less ground contact time and exhibit a mid-foot strike"
So, if your technique is reasonable, and you already include HIIT, consider adding Strength Training. Once you have a good strength base, then you can add Plyometric Training. (I included links to some lower extremity strengthening and plyometric exercises for running in this answer.)