Miles' wife had left him because after they had been together for a number of years, she realized that oddly, he was not aging. This was just too weird for her to handle, and, thinking that he had done something unnatural, such as perhaps sold his soul to the Devil, she left him, taking their two children along with her (Chapter 7).
At the time Miles' wife had left him, the Tucks did not know what the magic waters had done to them. They only knew that strangely, nothing seemed to be able to hurt them, and that while their friends and acquaintances were aging normally, they themselves were not. By the time they figured out that they had somehow been rendered incapable of growing old and dying because of the supernatural powers of the magic spring, it was too late for Miles and his wife. Miles, who should have been in his forties, still looked like he was twenty-two, not too much older than his own children.
When the Tucks figured out about the powers of the magic spring, Miles considered going out and searching for his wife and children so that they could drink the water and live forever too, but after thinking about it, he had decided that it would be no use. His wife would have been nearly forty by then, and his children grown, while he was still stuck in his early twenties. As Miles explains, "it'd all have been so mixed-up and peculiar, it just wouldn't have worked," so he just left it alone. When Miles tells Winnie about his family, he reflects that his daughter, whose name was Anna, would "be close to eighty now, if she's even still alive." His son, who is older, would be eighty-two" (Chapter 17).
Miles' situation illustrates the truth behind Pa Tuck's insistence that the powers of the magic spring be kept secret. Although the idea of living forever might seem appealing at first, the reality is that it would remove the recipient of this benefit from the natural stream of life, resulting in confusion and ultimate isolation.