Most "normals" in medicine are defined by averages of healthy people. There is usually a bell-shaped curve where most are in the middle, but higher or lower can be normal, as well. For resting heart rate, normal is defined as 60 to 100 beats per minute, but trained athletes can have resting heart rates as low as 35 to 40, which is normal for them.
The simple calculation for maximal peak heart rate is 220 - age for men, and 226 - age for women. That would put your peak heart rate at 176. For most people your age, your exercise heart rate would put them in an anaerobic zone (90-100% of maximum), which cannot be maintained for long periods due to the build-up of lactic acid in the muscles. However, this is clearly not an anaerobic rate for you. Based on the typical heart rate formula, your heart is acting younger than your 50 years.
Training more vigorously, pushing your pace into the anaerobic zone, will result in improved oxygen utilization, improved cardio-respiratory system, greater ability to tolerate lactic acid build-up, and improved endurance. Competitive athletes must do this to improve. There is also some evidence that interval training - fast for a few minutes, then slower for a few minute - burns calories more effectively than a steady, moderate pace. Only those people with coronary disease or structural heart disease need to be concerned about keeping their heart rate below a certain level.
Keep it up!