On an ordinary afternoon in March, Eliza, my 16-year-old daughter, plops her backpack at my feet, waves a brochure so close it grazes my nose, and makes a declaration that transforms my life.

"I'm signing up for the Marine Corps Marathon," she says. "I'll be running with a group that raises money for AIDS and trains Sunday mornings at 7."

"Seven a.m. -- are you crazy? Why would you pound your knees that way? Who can fit a thing like this into their schedule?" Then, pausing for less time than it takes to say "Power Bar," I add, "Tell you what -- if I can confirm with Dr. Ahlstrom, Dr. Gardner, and Dr. Steinberg that it's safe, I'll sign up with you." It is as though, for just this microsecond, I have morphed into Jane Fonda.

Back in my own, softer body, I confront different questions: Am I doing this for myself or for Eliza? Or to bolster my athletic image to friends and acquaintances? What if my 12-year-old takes up bungee jumping -- will I sign on for that? Am I willing to risk injury and, in turn, all the skiing and swing dancing that fill a void left by my separation from Eliza's father nearly two years ago? Isn't there a simpler bonding opportunity with Eliza? And an easier way to meet guys?

And why should I give up six months of Sunday mornings to arrive at my weekly training sessions along the C&O Canal earlier than the newspaper arrives on my doorstep? Surely, it's not because running 26.2 miles with thousands of other A-types has always been my dream; my dream is to get published in the New Yorker. More likely, my interest in becoming a marathoner could be called curiosity, the kind I have about what a hot flash must feel like.

Nonetheless, I attend an orientation meeting with Eliza where we sign up and exchange motives with other hopefuls. A trim secretary, seated beside me, tells the group, "My best friend is dying from AIDS. He can't run, so I'm doing it for him." Ashamed of my egocentric grounds, I sheepishly introduce myself as a blocked writer hoping, through running, to regain my focus. When Eliza announces that she looks forward to training with her mom and raising money, I rationalize that, through my offspring, I am exonerated.

At our first weekly training session, we are placed in pace groups with whom we will work out as well as run the actual marathon. Eliza's tight-abbed pack gets to line up near the front; despite our neon CoolMax costumes, my partner, Rayford, and I find ourselves in the rear among the less hurried. In the weeks that follow, the pain of placing one foot in front of the other is eased, ironically, by Rayford's sagas of his partner's death from AIDS and living with his own HIV. After we get through a 12-mile Sunday by exchanging the ordeals of Rayford's coming out and my final year of marriage, we agree on "Single in the Seventies" as our topic for the upcoming three-hour, 14-mile run.

If I were still married, quietly writing essays under a dogwood, I would have bristled at the idea of striding the equivalent of halfway to Baltimore (or if you compute all the training miles, round trip to Scarsdale). Back then, my exercise had a hedonistic spin: dreamily circling a skating rink or cycling with a lunch stop as the main event. Activity levels that require no shower. Isn't it striking how a major life change, like divorce, can transform you into the opposite of who you thought you were? Yet dim recollections suggest that the marathoner is who I was originally and that marriage molded me temporarily into someone who did a lot of sitting.

Sometimes I imagine Eliza and myself as two intersecting rings. I worry that I am treading onto her exclusive territory when I ask, "Would you mind if I try to keep up with your group on next week's six-mile maintenance run? It may be my only chance to jog with you before the distances increase." Even before she answers, her response is evident in her bright eyes, lit up the way they do on the towpath when her group, in its homestretch, passes me, still huffing my way to the halfway mark, and her fellow speed mates cheer, "Go, 'Liza's mom."

As Eliza and I plan a party for the fund-raising component of our marathon commitment, she asks: "Mom, how can I take credit for half the donations? They will be mostly from your friends." I tell her that so many of my friends are the parents of her friends and that we are in this together, a partnership. We shall not only jointly craft invitations and bake brownies, but we'll also tell what raising money for drug therapies that offer tremendous hope to people with HIV/AIDS means to us. I remind Eliza that without her, this expansion of my world would never have occurred.

As the training distances mount, I begin to believe that I may actually make it to the finish line. New queries surface. Will Eliza hang around on marathon day until I complete the course? Isn't it backward -- shouldn't the mama be the one to soak up her little girl's I-did-it grin as she crosses the finish line? Or is this one of those reversals dealt us by the passing years? On my birthday, Eliza holds out a cake she baked and goes "Yay!" when I extinguish all the candles in one blow. And on Oct. 24, there I'll be, sailing by on my merry-go-round as I cry, "Yo! Look at me!" She will jump and wave and cheer my victory -- and hers -- and ours.