I am 40years old, always been relatively fit. I have a heart rate monitor for running now. When running a 5k, my HR is pretty consistently at 185bpm for the majority of the run once warm up is complete. It is also the same give or take a few BPM when I do a 10k run. 5k time is around 25mins, 10k around 55mins. At no point during either of the runs would I describe my pace as at my absolute limit. I have checked manually and I would say it's pretty close, obviously hard to tell at 185bpm mid run. A. Is there something wrong with me B. Am I pushing too hard and need to slow down C. Does this maximum HR of 220minus your age actually apply?!