I need to be getting ready at 7:00pm as class starts at 7:30pm - was thinking of running mid-afternoon so I can maintain it even when clocks go back...
WWWFR, Not a HUGE difference, agreed, as long as it is still aerobic - once you go anaerobic there is a significant physiological shift. Heck, even I went over night, but still aerobic.
I guess you just have to watch it - if 147 is close enough to 145, what about 148? 150? And before you know it you are up to 155bpm. Fine for two minutes in a month to overtake a cyclist, but if becomes acceptable to you every other run, you are changing the structure of what you are doing.