OK How in GODS name am I suppose to do this?

I talked to my doctor yesterday like a good girl before starting an exercise program to maybe strengthen my heart and lungs. He wants me to exercise and keep my heart rate between 80-92 for 20 minutes.  So the first thing iI did was jump on my stationary bicycle and did about 3 minutes before my heartrate jumped to a nice 112. OK so maybe I should back off all resistance and try again.Did the same thing and made it to 4 minutes before I was at 117, maybe this isn't such a good idea after all. So I went to work and put a monitor on myself and started working. Just in the course of my everyday duties my heartrate stays at around 96-102. so how am I suppose to keep at my target Heart rate? This is so frustrating. I want to strengthen my lungs and heart but how am I suppose to do this if just normal walking gets me above the targeted numbers? And no way in He## am I going to be able to hold thiscurrent rate for 20 minutes solid without slowing down or stopping. My chest hurts way to much and I get to short of breath. Guess its back to the drawing board.