Avoiding overtraining is one reason to train with good heart rate monitor. In some days you must run much slower than other days because of stress, lack of sleep and rest. When you hit your upper limits, it's a sing to slow down.

Suddenly reduced HRV is another sign of overtrtraining. Good heart rate monitors (some Polar models with a strap) can do Orthostatic Test and give even more accurate information than heart rate.

https://www.polar.com/blog/heart-rate-variability-and-orthos...