Hi OP, I'm the same. I work out and keep myself in good shape, but tend to let my brain wander to thoughts like what if my heart just stops. I can't engineer a solution for myself, but I will be fully conscious of the fact that my body is failing me. It's terrifying, and you're not alone.

I don't have the right answers for you because I'm in a very similar position, but I can try to relieve parts of your existential anxiety. I used to think how crappy it was when you read on the news that some innocent bystander got shot or a freak accident occurred and someone died and how that could've been me. Ultimately, we should only worry about the things we have complete or partial control over. We are all susceptible to heart attacks, but we can also mitigate its probability through healthy diet and exercise. We are all susceptible to getting shot, but we can also mitigate its probability by choosing where we spend our time.

There will always be things we have no control over, but we should only concern ourselves with the things we do.

I would also recommend checking out "A Guide to the Good Life: The Ancient Art of Stoic Joy"[1] by William B. Irvine. It's been suggested on HN occasionally and it offers a philosophy on death as well as what I mentioned above (letting go of things you don't have control over).

[1] http://www.amazon.com/Guide-Good-Life-Ancient-Stoic/dp/01953...