I am 49 and a distance runner too. My resting HR is around 40 and I have huge trouble ever getting my HR above 145. 20 years ago I got it to 186 at the end of an all out mile. I ran a hard 9 miles 3 days ago and my average was 125 the whole way. On a bike going all out in a sprint on a slight grade I can get it around 145-50 maybe. HR is not always the best indicator of what is going on. Some the numbers I see posted are very different than mine. Heart rate does increase as distance goes up and fatigue set in.