Sub 20 is a very good time. I haven't looked it up, but a 19:59 would probably be in the top 10% of most 5Ks in the US with the exception of Carlsbad and the USATF Masters 5k Championships. It's probably not going to place in the top 3 in M40 in every single local 5k, but that doesn't mean that it isn't a very good time.
OP, I'd see another cardiologist and get a second opinion on running hard. You can't ignore potential heart problems, but OTOH, many doctors who don't work with athletes tend to have a knee-jerk "don't run" response to any illness. It sounds like you've found a happy medium between fanatical running and not running at all, but I'd want an opinion from a cardiologist who works with athletes or a very highly ranked cardiologist. My cardiologist has an entire floor of the hospital named after him AND he supports supplementation to maintain heart health. I see him twice a year.