Jeanne Louise Calment lived 122 years and 164 days [1]. Doubling the lifespan, 240+ years, is most certainly not possible, not reasonable, using diets, exercises, and so forth: if it were, we would have already been there. Even if we could push the body using current technology, no one will want to live 100+ years (from age 70 to 160+) as an old person, being in pain constantly, fighting all kinds of cancers.

Therefore, the only way I can see a lifespan push, not only x2, but x10, or even x100 (10,000 years—once you get x3/x4, it probably scales trivially) is through 100% organ regeneration and 100% programmatic control over the morphospace. Broke an arm? Amputate the limb and grow another one back, your cells already did it before. Liver cancer? Regrow another liver. Heart issues? Make another heart. Spend all the years above 35 as a perpetual 35 year old, eventually in the bodyshape of Chris Hemsworth/Margot Robbie (to keep the examples in Australia). This scientific fancy is somehow grounded in the mind-enlarging perspectives coming from Dr. Michael Levin's lab [2]. Personally, I believe enough in their thesis, programmatic control over cells, that I started learning about their work, BETSE, Bio Electric Tissue Simulation Engine [3], and try to build on top a BioElectric Simulation Orchestrator, BESO [4]. Most probably, however, we won't see 100% human organ regeneration in our regular lifetimes, but never say never.

[1] https://en.wikipedia.org/wiki/Jeanne_Calment

[2] Speaking with Cells: the Electrical Future of Regenerative Medicine with Dr. Michael Levin https://www.youtube.com/watch?v=RzGaakopAKU

[3] https://github.com/betsee/betse

[4] https://github.com/daysful/beso

You’re right we can push beyond that limit or even there (assuming that person actually lived that long, look up the controversy) with just diet or the biology we know today. But as you point that’s nothing fundamentally problematic with the idea of our body being viable for hundreds of years with the right support. So I’m hopeful.