Breathing then, If you’re breathing heavy at all. Back off.
I personally have a pace I can feel I can all day and not have any worries that’s my conversation pace. You can also use heart rate but I find my memory pace is pretty much perfect when I’ve really used HR before m.
I know this sounds silly, but how do you check your run is a conversational pace? I always run alone and never talk.
Is it just by feel?