The obvious thing here is diet. Have you changed yours? Not just on training days but generally. Think of the body not as muscles and joints but as systems, all working to keep you moving; the muscles produce a lot of toxins which need to be taken away by the circulatory and lymphatic systems to be processed by the lungs, liver and kidneys (mainly). This is all driven by the heart and your movement and controlled by your nervous system.
These all need to be working well and efficiently. Every thing you chuck down your gullet compromises these systems to a greater or lesser extent.
Diet is a hot topic and I’m no expert so look into it yourself. The only thing we know for sure is eating animals and their excretions is bad for you – keep this to a minimum.
And then think what else can you reduce; complex carbs, sweets, sugary things. All cause energy spikes. Necessary sometimes – near the end of a run for example but not every day.
And alcohol. My previous run averaged a 7:56 minutes per km. I’ve done no training since, not even on the bike but I haven’t drunk any alcohol. Roughly the same distance run and averaging 7:20.
What’s not to like?
Check out my run on Strava: https://www.strava.com/activities/1920403868/shareable_images/photo_based/6037999/1/d884a19d-fe47-4548-a6d8-5bf5411462d2?hl=en-US&v=1540204574&utm_source=org.wordpress.android&utm_medium=referral