Arthur Lydiard wrote the book Running with Lydiard about proper running technique, building aerobic capacity and nutrition. I have started practicing according to his method and was able to run 18km without any pain. Strongly recommended for every runner.
To become an experienced runner may take few years, I'm doing similar amount of run plus gym 3-5 times a week.
I assume you are running not just for fun but to be able to run at least half marathon.
To protect and improve heart muscle I do recovery run on low heart rate 120bpm which is baseline for any type of runner as a recovery mode run. 30 minutes which is equal 4-5km that pace is enough. If you look any marathon training program, there is a common pattern 3-5km run during a week plus long run over weekend. An increase may be around 10% per week.
Personally I don't run longer than 6km now that's why I run on "safe mode" only to help body to adapt and reduce risk of injury. I did many mistakes like hill strikes and know more what right running technique is. The 120bpm is safe zone where you can run longer, detect many type of issues, learn how to breath and reduce heart rate. No marathoners run a long distance on higher pulse, it is just not safe.
I can recommend only what helps me: be consistent, take your time, listen your body, let it adapt, do warm up and cool down every single run.