The idea is that he doesn't have the time to explore in depth a drive through Spain he made decades ago or a dairy farm in Indiana with 25,000 cows or any number of other ideas he's been saving up to explore, so he is dipping his toe in them, then moving on.
McPhee is basing his project on a long-ago encounter with a 66-year-old Thornton Wilder, who had decided to catalog all 431 plays of the playwright Lope de Vega. The younger McPhee didn't understand why Wilder was doing this. The older McPhee does: it's a project without an end, a way to keep yourself going.
This got me thinking about what I do, am doing, to keep myself going, specifically my writing self. And the answer, right now, is simple: Every day, I write a blog post. And I've written one most every day for close to ten years. A Walker in the Suburbs is my Middle-Aged Woman Project.