Nope, I don't worry too much about my heart.
But I didn't worry about cancer until it happened to me. Mine was taken care of with surgery, no chemo. So far, so good, no recurrence.
I've had occasional cardiac oddities for as long as I can remember, and I've been physically fit most of my life. Various doctors at various times have mentioned a murmur, afib, arythmia, etc., but nothing ever shows up consistently on tests and none of 'em seemed too worried about it, including when I described my fitness and diet routines. Looking back and more recently at those occurrences of irregular heartbeats, etc, it usually coincides with stress -- anxiety, overwork, inadequate rest. Occasionally my BP and HR will spike for no particular reason, but usually it's within the normal range.
I'm pretty careful now about my fitness routine, including giving myself plenty of time to warm up. I need about an hour to warm up before my heart rate and blood pressure stabilize and I don't gas out on sprints or climbs. I join a few somewhat fast group rides a month, but if I'm not feeling it I'll drop out and go my own way at my own pace. On good days I can keep up pretty easily. A couple of weekends ago I had both experiences -- gassed out on our first cold morning, couldn't seem to get my legs working, couldn't keep up with a moderate 15 mph pace. The next morning I was fine with a 17-18 mph pace on a 35 mile ride. My only competition is the calendar and nobody beats that game. So I just do my own thing and wait for a better day.
My family doesn't really have any serious history of congenital heart defects or cancer. Most of the cardiac and respiratory maladies were related to smoking, poor diet and lack of exercise. Generally they withered away in their 70s and 80s from congestive heart failure.
A great aunt died of leukemia. My dad died after a long bout with prostate cancer that suddenly metastasized after we thought he'd beaten it. More than 10 years without too many problems, then he was gone in a month in his late 70s. But that's the only occurrence of cancer metastasizing in my family, that I can think of. A couple other folks had benign tumors, melanoma, nothing too unusual or too frequent considering the size of our large extended family -- I'm thinking of dozens of relatives and only a few instances.
I'm more worried about dementia and Parkinson's. Those occurred on my mom's side of the family.