Zone 2 is the easy run that makes up most of a week. Guides often say 60 to 70 percent of max heart rate. The problem is the max. An age formula can miss your real max by 10 beats. A wrong max makes an easy run look like a workout, or the reverse.
The talk test
If you can speak in sentences, you are in the right place for an easy day. If you can only get out a few words, it is no longer easy. That test does not need a watch. The form guide is the next place to look if the easy run still feels forced.
If you have a tested max
Use that max, not 220 minus age. Keep easy days under the point where talk breaks. A race pace is not an easy pace. The workout glossary separates easy, tempo, and intervals so they do not all become the same run.