Jenny sounds correct, the action is called "oxygen debt", usually if you continue your exercise, eventually your larger muscle groups (legs) will get enough O2, then your heartrate will drop back down somewhat.
It is not unusual for your heart rate to raise suddenly at the onset of physical activity. There is an initial rapid HR increase in order to meet the increase in the bodys demands for oxygen. It may be worthwhile carrying out a less strenous exercise such as brisk walking and seeing if your heart rate settles.
Also 147 doesnt sound to high.
Heart rate vary's from person to person. My suggestion is that you see a cardiologist and have a workup if you have never had one. You are 48 and should have a treadmill stress test.
It is difficult to determine your conditioning level based upon what you said. Conditioning level has everything to do with your heart rate. I'm 50 and have an exercise heart rate of about 140 when running 3 miles.
147 isn't necessarily too fast. The question is what is your resting heart rate, and how fast does your heart rate come down after exercise? Recovery rate determines conditioning level.