IN HER WORST days, the days when she saw life as something that could arbitrarily be snatched away, Mary Louise Zeller would be strolling to the park with her little boy. Suddenly, out of nowhere, the thought of a car would come careening into her brain. As clear as anything Zeller would see the car jumping the curb and running them over.

She was afraid of everything then. When she slept she had nightmares. In the daytime she was on guard. Washing dishes at her kitchen window, she would mentally dodge bullets that hadn't been fired from a passing car.It was a free-floating anxiety that threatened to submerge her and her family. "Mary Louise, you're going to ruin this child," a friend who was a psychotherapist told her, seeing the way Zeller hovered over her toddler. You have a classic case of post-traumatic stress syndrome, her friend said.

Two months earlier, Zeller had watched in horror as 19-month-old Adam had fallen from a second story window, head first onto concrete. Incredibly, the boy survived and recovered. But the accident seemed to create a new fissure in Zeller's brain and trapped a thought there: You're vulnerable, it said; in a split second you can lose everything. And, indeed, a few weeks after the accident, Zeller had a miscarriage.

So by the time her psychotherapist friend suggested Zeller needed to get out of the house and spend four hours a day away from Adam, she had totally lost her courage for life's daily uncertainties.

Perhaps that's why the ad she read in her local paper caught her eye. Learn self-confidence, it said. Learn ways to defend yourself.

She didn't really expect to sign up for the tae kwon do class. She just planned to inquire about the lessons and then leave. She was 46 then.

Now she is 51 and transformed by martial arts, she says, into "a modern-day warrior." She is a second-degree black belt, has won 15 gold medals in state competitions and took a gold in full-contact sparring at the 1994 national championships.

Now she's shooting for the 1996 Olympics.

"I'm going to make a declaration to you," she tells the reporter. "I'm not only going to the Olympics, I'm going to take a gold medal at the Olympics."

It is Saturday morning at the Smith Field House at Brigham Young University, one of Zeller's training venues. She is dressed in traditional white tae kwon do garb. Around her waist is the hard-earned black belt, inscribed with the name "Mrs. Mary Louise Zeller."

At 51 she regularly spars with men and women half her age; even teenage boys, their arms and legs exploding with energy and fearlessness. At matches she competes not only in the 35-and-older category but in the younger division, too. Last spring, at the state competition in her home state of Georgia, she took first place among everyone 17 and older in her weight category.

At the Olympics she would be the oldest person ever to compete in a sport that requires you to sweat from something other than fear. Older people have competed, including two older women, but the sports have been more sedate: equestrian, archery, shooting.

Zeller has a 29-year-old son and a two grandchildren in addition to 7-year-old Adam. She is quick to capitalize on the image of a grandmother as black belt, and the media is happy to oblige.

"Ninja Grandma," the Tae Kwon Do Times headlined in July. "From Worn Out to `Warrior,' " another magazine promoed on its cover last month. A Salt Lake TV station did a story, too.

Zeller knows it is her age that will get her noticed. But if she wants to make the Olympic team, she'll have to train and fight as if she were 20 again.

She will be sitting by the ring at some competition, waiting for her chance to spar, trying to work up enough courage. That's when she'll start philosophizing with herself about the notion of time.

"It's all made up," she says. "We have this whole culture about being middle-aged. We think this is the way it has to be."

So, sitting there waiting for her turn to possibly be kicked in the head, she comes up with her own definitions.

"This is a game we play," she tells herself. "I made up being a middle-aged housewife. So right now I'm making up being something else. I'm an outlet for the champion spirit. Who I am is a modern-day warrior."

The "warrior spirit" is central to the Korean martial art of tae kwon do. A true warrior, says Zeller, has an inner strength as well as toned triceps. A warrior gets discouraged but doesn't give up. A warrior stays focused.

Focus was what she was definitely lacking, her teacher told her when she answered the newspaper ad that very first day five years ago. Her teacher - later to become her "master" - asked her to make a fist and punch it straight in front of her. Zeller's arm wobbled. "It's your life force," Master Kim told her. "You need to learn to focus it, use it, project it."

That night, after an hour of blocks and punches, she went home sore. In the months that followed she would sometimes be so tired she would fall asleep in the bathtub. She would be so sore she would have to bounce on her bottom to get down the stairs.

That was in California, where the Zellers lived before Ron retired from his job as an educational consultant and the family traveled the United States in a mobile home, stopping to settle down everyone once in a while and enroll Mary Louise in some tournament. This year they bought a house in Orem, where Zeller plans to set up a studio with her new master, Scott Chun.

Although she seems quite California now - a person who hires visualization coaches and acupuncturists - Zeller is a Southern girl, raised in Atlanta.

"When I was growing up Southern, only the sluts would fight," she remembers. "It was low class." And never raise your voice, Zeller's mother told her.

So now her mother, the one who trained her to be a lady, will sometimes watch her compete in "forms" - tournament exercises that showcase a participant's style. But when the sparring starts - the fighting and the yelling that goes with it - that's when her mother goes home.

Within six months after she began learning tae kwon do, Zeller felt like a different person, she says. Partly it was the repetition of movements, she believes. Wax on, wax off, as Mr. Miyagi said in "The Karate Kid."

"I think it develops neuro-pathways in the brain for focus and concentration," she says about the hundreds of jabs she would do over and over.

She figures she looked stupid at first, poking and kicking at the air, doing it all wrong. "They say white men can't jump," she says. "But you haven't seen nothing till you've seen white women jump." You have to be willing to look stupid, she says.

"If you do it about 100 times stupidly, about the hundredth time you'll get it."

Zeller stands in front of the mirror at the field house, bouncing on her toes. Later she practices kicks with the boys in her class. Ki-HAP, she yells, as she fwaps her foot against a body shield. Ki-HAP, fwap, Ki-HAP, fwap.

She does 500 kicks a day but wants to do 2,000. She runs two miles, does sprints, lifts weights and spars. For endurance she takes "adaptagens" - a Russian herb concoction she gets from her sponsor, a multilevel company called PrimeQuest. The adaptagens are a natural alternative to steroids that have quadrupled her strength and her ability to heal, she says.

By spring, she will be training at least six hours a day. If she wants to make the Olympic team, she says, she'll have to work harder than she's ever even thought of working in her life.

She will also have to win at least a third-place medal at the tae kwon do national championships in May in Colorado Springs. Then she'll have to do well in the team trials. And before all that she'll have to place at the Utah state tournament in March.

The first time she went to a tae kwon do tournament, in 1989, she thought Master Kim had just invited her along to help drive. She hadn't really expected to ever compete. By her third tournament, when she was only a blue belt, she won gold medals in both forms and sparring.

To date, Zeller has never lost in the 35-and-older division. Last year, at nationals, she not only won a gold in forms and full-contact sparring - in the 17-and-older category for middle weights - but in high-jump board breaking.

But she didn't make the Olympic team in 1992. Her opponent at nationals that year psyched her out, she says.

"You're not going to hurt me, are you?" the woman asked Zeller right before they entered the ring. Like a well-placed block, the question threw Zeller off-balance. She held back, and she lost.

"I was never as disappointed over love as I was over not making the Olympic team," she remembers.

This time she fantasizes about doing a Roger Bannister kind of thing. Bannister broke the four-minute mile barrier when no one thought it was humanly possible. Since then, people run the mile faster than four minutes all the time. If she could get to the Olympics, Zeller says, she might show other women what is possible, too. Not necessarily martial arts, not necessarily the Olympics. But something done with passion.

"Find a game in life worth playing," she says, "and pull out the stops."

Tae kwon do has given Zeller three jaw dislocations, a chipped tooth, a fractured knee cap and a broken toe.

She knows the statistics: At the tae kwon do nationals in 1993, out of 2,000 competitors, 500 made trips to the emergency room. Three ambulances waited outside the tournament hall, just waiting for the next injuries.

It could make a person too afraid to really jump in and try her best. But giving in to fear is no longer an option.

"You exercise your courage, just like you exercise your muscles," she says. After four years of punching and being punched at, of pushing her body farther and faster, of getting back in the ring after injuries and losses, Zeller no longer sees doom at every turn, even though the risks are even greater now.

This is what she can say now, as serenely as if she were talking about a stroll to the park: "Life is dangerous. We live with obliteration in a thousand different ways."

On Saturday Zeller will be in California, competing in an all-California championship. Sitting at ringside, waiting for her turn, Zeller will remind herself that she is a warrior now.

And she'll remember the advice of the fictional Mr. Miyagi to the Karate Kid: Do karate yes, or do karate no. Walk down left side of street, yes. Walk down right side of street, yes. Walk down middle of street, you get squished like grape.