training after a heart attack; building intensity

I am not yet a Masters swimmer, but I hope to get involved in the next year or two. In the meantime, I work out on my own. About 15 months ago, at the age of 44, I had a heart attack. They put four stents in my heart. I have susequently been checked out via catheterization, and my coronary arteries are clear. I now take a variety of medications. Once upon a time, I was an okay freestyle sprinter. My cardiologist tells me I should work out at "moderate intensity" which I find hard to do. But I warm up slowly and cool down slowly, too. In between I am doing pretty modest intervals (100s on 1:30, 200s on 3:05, 400s on 6:10) and am feeling encouraged because I am getting stronger. That said, sometimes I feel my limits and cut the workout a bit short. (I go around 3000 yards a workout approximately 4 times a week.) Here is my question: is there anybody out there who knows enough about coronary heart disease and competitive swimming to give me any guidance? Can I keep pushing to go a little harder, or should I back off? I would really like to compete again. Thoughts? Many thanks! Red60