Best way to injury and poor performance is trying to do a PB every time you go out for a run.
Do 90-95% of your training at 75-85% of your maximum heart rate – this may seem like an easy pace but you will get much, much, much faster. I’d certainly do at least 4 weeks of 3 runs a week without leaving this intensity to create a good running base, get your muscles used to the impact and improve base speed. Like I said, it probably feels too easy but you will be doing your running the world of good.
Just pop into 90-95% of your maximum for short durations during efforts (30s -60s).
If your running at 86-89% of your maximum heart rate then, possibly, the only good you’re doing is familiarising your muscles with the impact of running and not doing much else – probably better to sit on the couch rather than to fatigue yourself. These are known as ‘trash miles’ – miles for miles sake with no improvement or advantage.