How hard can I push myself without suffering a heart attack?

Hi all,

I am a 50 year old male who is a bit of an exercise nut. I have a resting pulse of 55, and push it up to 160 during a strenuous work out on my cross trainer. I know that I'm very fit for my age, but I'm sort of worried that now I'm 50, I might just overdo it and keel over. All the men in my family for the last few generations on my Father's side have dropped dead of a heart attack in their 50's and 60's, though my father, who's 80, is still going strong despite suffering a stroke six years ago. My mother, who's also 80, underwent a heart bypass and aortic valve replacement two months ago.

Can anyone give me any guidance about whether I should just keep on as I have been ( I feel fine by the way ), or throttle back a bit, or, should I just listen to what my body is telling me and forget my age. Thanks!