It really depends on the day. Some days 9:15 feels easy and other days, like today, I am going along at 10:15 and it feels tough.
Blog Time! From the Kitchen to the Road
Day to day- as others, early morning and half awake vs midday and peppy, weather, phases of moon, randomness- pace varies about 30 seconds for most, but can be up to 45-50 seconds for the outliers. Now that I think about it, it would probably be a bell curve, so most runs are within an even more narrow range. Say standard is 8:25-8:35, but peppy days I can run easy and wind up at 8:15 or even 8:10. Other days it's closer to 9:00 or even a few seconds more. They key for me is I jut run and check at some known mile markers or even at the end.
Within a run, without trying I wind up being very consistent (hilly routes change it up of course).