Four years ago I went to the Horniman Museum one August afternoon, and afterwards I sat in the museum’s garden, and wondered how much time I had left. I was forty-one, out of­ condition, out of sorts, unemployed and unattached. And would you have said, in the annoying way that people have: snap out of it, get some exercise, get a job, get hitched, what are you talking about, you’ve got all the time in the world? I sat in two places in the garden. It is on a hill, and has views over London that are called commanding. In…