This may sound overly simplistic, but do you exercise? I’ve been surprised by what a difference exercise has made in my life over the last few years. It doesn’t make sense that exerting more energy would give you more energy, but it’s somehow often the case. It doesn’t have to be running long distances or anything. Walks, dancing, stretching are all lovely. My father—in his mid-seventies—has exhibited the same thing. He does yoga and a lot of walking, and he is more energized now in retirement than I’d ever seen him before.