It's hard to disagree with living a long and healthy life, although if it's only to 65 then I'd probably rather be smoking and boozing. Give me 85 at least!
Studies have a habit of contradicting themselves, but the point of the twin study is that longevity, unlike size, eye color, hair color, or to some extent athletic ability, is not controlled by a pattern of genes that we understand yet. There is no set of genes which guarantee long life, and the variation between twins in life outcomes is huge.
In any event, I'm reminded of CS Lewis's logic on whether one should be religious. He said that since God's existence was unprovable, it was the safer option to believe in God since if God exists, one would be saved, whereas if He doesn't you're dead anyway.
Similarly, choosing fitness is going to be the safer choice (as long as you don't sacrifice all of life's enjoyments for it). Spoken like an Irishman, right Mike?