I had a heart attack at age 44 - similar to you, I couldn't believe that it could be that since I was way too young (I am 52 now, and still way too young ). My wife is a nurse, and when I described my symptoms to her, she had me in the Drs office before I could finish my sentence. I too ended up with a cardiac cath and stent, mine is in the LCX.
During the cardiac rehab, I spent a lot of time on exercise bikes and treadmills with a heart monitor attached. They told me that they wanted me to be able to feel the amount of exertion that it took to bring my heart to a particular rate. I was told to exercise with my heart rate at ~135 to 140. I started cycling after that, and got out on the road, and spent a lot of time riding before I got a heart rate monitor, and what I found was that for the same perceived level of exertion, my heart rate was much higher when riding on the road.