Surely it's just a genetic lottery win? Humans certainly aren't designed to live that long and people with perfectly respectable lifestyles drop dead in their 50s and 60s every day.
I am not sure whether the secret for longevity lies in the genes or in the lifestyle. But you can't say that
humans aren't designed to live that long. There are references from 400 or 500 year ago, about people living up to 100 year or more. As the living standards increase, along with the advancement in the medical science, longevity will increase further.
There are some physiological limits. Approximated limit of average lifespan is about 250 years. If we solve genetic issues then there still would be another limitation due to nature of our brain. To be exact, there is no possibility to remember more than 4000-5000 years of living experience. You won't die but you'll forget anything what happened before last 4000-5000 years of your hypothetically unlimited life. And there will be no way to recover these memories.