Doing the Right Things Does Help
We can never know absolutely and for sure whether doing the "right things" for our health will make a significant difference to our own healthy longevity. You have to wait and see, one chance to get it right, no going back to fix things up. We do, however, have a wealth of evidence that actions long commonly regarded as the "right things" for good health will indeed be good for our future healthy longevity. This evidence is quite separate from the comparatively recent investigations of medical science into the biochemical roots of good health and longevity.
What is this evidence? That wealthier, higher IQ people tend to live longer and suffer less age-related illness. For example:
Lower scores on measures of IQ at two time points were associated with [cardiovascular disease] and, particularly, total mortality, at a level of magnitude greater than several other established risk factors.
I don't think that it's ever been a grand mystery that regular exercise, a good physician relationship, and eating sanely are going to be good for you; the common wisdom for good health long predated the scientific studies showing that it was the case. The grand mystery is why so few people keep up with those efforts in their own lives, and suffer because of that negligence. I've been inclined to interpret results like the research above to mean that more intelligent people tend to get wealthier but also tend to do more of the right things for their health - you can be as rich as you like, but if you weren't exercising all that time you were making money, you're still going be at a higher risk for suffering cardiovascular disease at the end of the day.
Smarter people have a greater tendency to keep up with common sense health practices and gain a benefit by doing so. That's my thesis. As to why that is the case - well, that gets back to what IQ actually measures, whether time preference is very different between individuals, and so forth.
A complicating wrinkle is that chronic illness affects the brain. The chronically ill--including those who are in denial, misdiagnosed, undiagnosed, or still sub clinical and therefore unaware--can have their mental abilities sapped by their sometimes subtle neurochemical imbalances. This can manifest in a number of ways. Overly emotional decision making is one, as the emotional centers of the brain become overly excited and override the logical centers. The forgetfulness, confusion and inability to concentrate that sufferers descriptively call "brain fog" is another.
So the same underlying condition can drive both mental and physical deterioration, and a downward spiral can result as both types of deterioration compound each other. IQ and personality factors still help, though. An already powerful mind will be better equipped to work around a partial loss, and a smart and responsible person is more likely to have good habits in place prior to falling ill.