I think that while exercising and trying to eat a good healthy diet is important for everybody to do, I think that genes still play a more important factor in regards to living longer. There are a lot of cases where people who exercise and eat well still get diseases and idea earlier than expected.
Diet does affect health, on both sides (you can starve yourself to death, or die of nutritional
deficiencies, as well as eat your way into Type 2 diabetes). And genes
play a role in cardiovascular disease, diabetes, and all the other
diseases of an agricultural society. You can feed two people the same diets and get vastly different results, based on different genes.
Regardless of how healthy of a lifestyle you have your genes are a huge factor in how long you are going to live, if your family has had a history of heart disease or cancer you have a much higher likelyhood then average of getting it yourself regardless of what type of healthy lifestyle you lead.
A healthy lifestyle usually wins out because the environmental circumstances can play a major role in the way genes flourish or act in a given environment. Even the best of genes can be ruined by an unhealthy lifestyle, but if someone has bad genes, they can typically overcome this with a healthy lifestyle, or they can lead a healthy lifestyle in a way that influences the way genes are passed down into a new generation.
Personally I think genes and a healthy lifestyle contribute to a long life, but when forced to pick I would say a healthy lifestyle makes the biggest difference. A person can't really control or manipulate their genes. If their life is going to be cut short due to genetic issues there's not much you can do to change. On the other hand a healthy lifestyle is a choice, so your genes might say you're going to live long, but if you don't take care of yourself, you're negating those chances.