It's what you are used to and what your body can handle.
I've just been reading about running slower for better results. Maybe I need to aim for a lower heart rate on some of my runs. Something to try this year. Will still do some intervals and parkrun is my gauge of how I'm doing.
There's certainly something to be said for not maxing out, I made a point of deliberately not pushing it today when I could have gone faster, I noticed I was already 30 mins faster than my same run last week, that's enough of a gain for one week!