the lord of the rings - How did Gerontius Took get to be so old?

Until Bilbo turned 131, Gerontius Took (AKA "The Old Took") was the longest-living hobbit ever, reaching 130 years-old.

Bilbo had The Ring to help extend his life, and even then, his age took a huge toll on him later in his life.

Gerontius, however, didn't have The Ring to help him. Is it ever explained how he was able to live that long with out it?

Answer

There is no in-universe explanation given for the extreme age of the Old Took. Absent any other information, we have to assume that he was just lucky, which has been known to happen, even in our world¹.

Interestingly, Gerontius' age isn't totally unique; after him, the oldest (non-Ringbearing) hobbit is Lalia Clayhanger, also called Lalia the Fat, his granddaughter-in-law. Although she died at the age of 119, her death was accidental; Tolkien reports in Letter 214 that she was in remarkably good health, and had a number of good years left in her (bold is my emphasis, italic is Tolkien's):

A well-known case, also, was that of Lalia the Great (or less courteously the Fat). Fortinbras II, one time head of the Tooks and Thain, married Lalia of the Clayhangers in 1314, when he was 36 and she was 31. He died in 1380 at the age of 102, but she long outlived him, coming to an unfortunate end in 1402 at the age of 119. So she ruled the Tooks and the Great Smials for 22 years, a great and memorable, if not universally beloved, 'matriarch'. She was not at the famous Party (SY 1401), but was prevented from attending rather by her great size and immobility than by her age. [...] Lalia, in her last and fattest years, had the custom of being wheeled to the Great Door, to take the air on a fine morning. In the spring of SY 1402 her clumsy attendant let the heavy chair run over the threshold and tipped Lalia down the flight of steps into the garden. So ended a reign and life that might well have rivalled that of the Great Took.

^{_{The Letters of J.R.R. Tolkien 214: To A.C. Nunn (draft). 1958/59}}

Out of universe, Gerontius' age was inspired by Tolkien's own grandfathers, who each lived to be nearly a hundred. Tolkien revealed as much in an unsent letter to a Mr. & Mrs. Kloos, excerpts of which were printed in A Reader's Companion:

[The Old Took] has part of his origin in the fact that both my grandfathers were longeval. My father's father was in his eleventh year when Waterloo was fought²; my mother's father, a much younger man, was born before Queen Victoria came to the throne, and survived till his ninety-ninth year, missing his 'hundred' (with which he was as much concerned as Bilbo was to surpass the Old Took) only because he mowed a large lawn that spring and then sat in the wind without a jacket.

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¹ Although 130 years is a bit beyond even the longest-lived of modern humans

² According to a copy of the Tolkien family tree (PDF link) posted on Hammond&Scull.com, John Benjamin Tolkien lived from 1807 to 1896, dying at the age of 88 or 89 (depending on what month he was born). I'm not sure how he was in his eleventh year during the Battle of Waterloo, but Tolkien's dodgy maths is a topic for another time.