Do yourself a really big favor. Ask your doctor for a heart stress test. It's the only truly safe way to find out what your limits are (heart wise, anyway) and if something bad is going to happen, that's the place for it.

After years of century rides, racing, and daily commutes, I had a heart attack at 42. All along, I had a heart artery with a partial blockage and never knew it. I now have a heart stent, and at 65 years old, still ride, but I watch my speed and climbs now a lot more closely.