Bodies react not always the same to workouts and to measure the training intensity you need a heart rate monitor. In this way you can control your various workouts perfectly.
You don’t go too fast in your workouts, and not too slow. The same is true for races.
Furthermore you can perfectly control your shape, because you i.e. have an average heart rate of 145 instead of 155 at a 10 km (6 mi) run in 50 minutes.
Many heart rate monitors have a distance-tracker and other comfortable functions for your workout control.