早在“纳艾德”,安妮特·贝宁long-distance swimmer Diana Nyad, is staring into a mirror. She has just turned 60 and has a look of dread and stupefaction on her face. It’s a look that expresses a feeling that everyone of a certain age experiences: “I am slowly becoming an old person. I am being sucked into a vortex of genericness.”

What Diana does with this feeling becomes the story of the movie. After 30 years of not swimming, Diana decides that she is going to do something that she’d tried and failed to do at 28. She will swim from Cuba to Key West, Fla., a distance of over 100 miles that will take at least 50 hours — a feat either beyond or right at the limits of human endurance.

So “Nyad” is about someone trying to do something very specific. It details how she got into shape, assembled a crew and attracted sponsors for one bold attempt, and then another, and another.

But in a larger sense, this movie is very much and quite consciously about something more universal, namely what it’s like to be in one’s 60s. It’s a weird time of life: You feel good. You feel like your old self, but you know that something could easily go wrong, and that even if you’re lucky, you probably shouldn’t embark on the writing of a 20-novel cycle, at least not if you intend to finish it.

So what are you supposed to do? In “Nyad,” Diana specifically asks that question. To her mind, the world is expecting her to “shut up, sit down and wait to die.” But she keeps thinking of the line from the magnificent Mary Oliver poem, “The Summer Day”: “Tell me, what is it you plan to do/ with your one wild and precious life?” Does that question, which anticipates something wild and precious in the future, still apply to someone in her 60s? She decides yes, but it’s not an easy yes. It’s a yes that takes work.

What emerges is a unique portrait. Bening plays Nyad as a relentless personality, someone who has a hard time shutting up, who talks too much about herself, who is selfish and demanding and prone to lashing out in anger. But then again, no normal, reasonable person would do what she does.

The movie explores the real essence of determination, and it’s not what people imagine as they recite affirmations to themselves. “Nyad” shows us determination almost at a level of pathology, as a single-mindedness that could be considered sick, except that Nyad wasn’t delusional about her capacities.

For sure, this steamroller of a character provides one of the great Annette Bening roles — she doesn’t get enough of them, but when she does, wow. Her Diana Nyad isup there with her work in “Being Julia”and“The Kids Are All Right.”

Her intensity is nicely balanced by Jodie Foster as Bonnie Stoll, Nyad’s best friend and swimming coach. As Bonnie, Foster is charming, thoughtful and fairly passive, but not a doormat, and when we want to know how we should feel about Diana, we look to her.

“Nyad” was directed by Jimmy Chin and Elizabeth Chai Vasarhelyi, who specialize in adventure documentaries. The movie spends a lot of time with Diana in the water, and the filming of those long scenes is meticulous and visceral. You really feel that Bening is making that journey. You might start to feel that you’re swimming with her.

But the movie’s technical excellence is overshadowed by its human dimension. It’s a strange thing that an act of physical endurance that accomplishes no larger purpose should be so inspiring. But some achievements become bigger than themselves by somehow standing for something intangible, such as the importance of challenging oneself, and of being alive while you’re alive.

In making those attempts, Diana Nyad knew she was giving a gift to people, and “Nyad” passes it on.

Reach Mick LaSalle: mlasalle@sfchronicle.com