I missed this when originally posted, too. But like several others, I can speak to this firsthand. Things can happen in an instant, there is no way to prepare for it. Tomorrow is never a given, so cherish every single day and treat it as a gift from above.
DH passed unexpectedly from a massive heart attack on Mother's Day 2014. No signs, no symptoms, sitting on his buddy's tailgate holding the hose, while his friend finished detailing his boat prior to launch. He'd had minor surgery 4 months prior, and had clean bill of health from his doctor (who re-read his chart numerous times, looking for any tell-tail evidence), and all his pre-op tests -- stress test, EKG/ECG (can't remember which), chest x-ray, the whole gamut. No cholesterol issues, no blood pressure issues, nothing, and he was physically fit, not muscular, but he ran roughly 3 miles every day and the occasional 5K for charity. Here one minute, gone the next. Tell your loved ones how much you care for them, make time to visit with your family, make amends if that's needed. It's real, it happens, you cannot plan for it.
And, I will add this -- make sure your loved ones know your wishes re: organ and tissue donation. The hospital he was taken to was a small, but well equipped, non-university hospital, so no live donation of organs, only what could be taken after-the-fact. There's nothing worse than being asked on the phone, within hours of losing your loved one, if you want to donate corneas, tissue, bone, etc.