In reply to Ffati1:
I generally use RPE rather than heart rate. For the limited amount of road running I do, I also use pace. HR I find most useful for calibrating effort occasionally (if it feels easier or harder than I expect) and for post-run analysis to monitor progress. Getting familiar with RPE will help a lot with pacing races (especially ultras if that’s your plan).
For RPE, as others have said you should be able to comfortably hold a conversation if running easy (HR zone 1 & 2 in a 5-zone model). Another way of calibrating is to try eating - especially helpful if training for ultras - if you’re struggling to chew/nearly choking on an easy run, you probably need to slow down.
For setting HR zones, I use lactate (anaerobic) threshold HR, which is roughly between 10k and HM pace for most. And if using HR, make sure you’re measuring it reliably (so not wrist OHR for most people, either chest strap or - what I use and recommend - an optical arm band like the Polar Verity Sense).
Be wary of YouTubers etc claiming to offer magic bullet training or demonising particular efforts/zones. You need to maximise consistent volume, which means a lot of easier running. And ideally your quality sessions will be focused on the demands of your planned distance/race - in terms of effort, climbing, terrain etc. Depending on your planned distance this may mean a fair bit of time in zone 3!