Heart rate is personal. We all have different max and min. A good example would be Frank Shorter's resting rate was high for a World Class Marathoner. Bill Rodgers heart beat just occasionally. when at rest. Both were better than any Americans since.
In my peak (around 45) I took my resting pulse each day before getting out of bed. It was a natural reflex. For me 42 meant I was rested and ready. 50 might mean a cold was headed my way, or that I had not recovered from the training the day before. My max then was only about 185. I am not sure about now, but have seen it already at 169. I am not too concerned as to the max/min now, but in a few months, will again tune in on it. I know if I am working, and I know if I am working too hard. Now I am working too hard from necessity, but I always did train and race a little closer to my max than my peers. I had a lower max HR than many, but could work at a higher percentage of it for long periods.
Basically you race distance at your aerobic threshold. That is the line where running above kills you in a distance race. I will work on that line in a few months. The key in training is to raise your aerobic level as high as possible. Right now I desperately need a base which is gained by endless slower miles.
Any fool can run himself into the ground. The key is too train with intelligence.