With just a little experience at regular fast running, you can tell from your internal sensory data if you’re working at the right intensity. So while heart rate monitors can provide additional support to your feeling that, yes, this is an effort level I could sustain for an hour, they’re not necessary on those runs for most people.
Instead, the best use of a heart rate monitor for most runners is to make sure you’re not going too fast on your recovery days. When you’re used to plugging away at about the same effort level most days, consciously keeping things below 70 percent of your maximum heart rate can feel absurdly slow. But that seemingly too-gentle level of effort is what you need after a hard workout to truly recover. If you have the common struggle of running your easy days too fast, set a heart rate monitor to beep if you go above 70 percent of max. If it beeps, slow down until the beeping stops.