LOS ANGELES MARATHON : At Last, the Carrot Is Plaatjes’ : Race: He leads most of way after failing three times. O’Brien breaks the women’s record.

How long should you have to wait and work for something before it becomes yours? Is it right or fair to dangle a prize before you, only to snatch it away at the last minute--three times?

This is how it has been for Mark Plaatjes since he came to the United States in 1988 with his wife and daughter from South Africa. The Los Angeles Marathon was the first race Plaatjes ran after being granted political asylum. But the race had done little but break his heart--until Sunday.

Plaatjes took the lead after six miles and was not challenged in winning Sunday’s sixth annual Los Angeles Marathon. His time of 2 hours 10 minutes 29 seconds was 10 seconds off the course record, but to Plaatjes--who has won only one marathon in four years--the victory was an achievement of the highest order. And it came at a time of personal anguish, with the news of the serious illness of a brother who raised him.

It was a breakthrough performance for the women’s winner, Cathy O’Brien, whose time of 2:29:38 set a course record, bettering Julie Isphording’s 2:32:35 of last year. O’Brien also broke the 2:30 barrier, the benchmark for elite women marathoners.

Plaatjes and O’Brien each won $55,000 and an automobile. O’Brien also received a $10,000 bonus for breaking the course record.

Both winners’ times are all the more impressive in light of the manner of their victories: leading nearly the entire race, setting the pace and doing all the work on a warm and humid day. A Los Angeles Police Dept. spokesman estimated 250,000 spectators lined the route. Race organizers were unable to provide a figure on the number of entrants, but--as always--the course was clogged with thousands of runners.

Plaatjes admitted that at the start he was concerned about the heat--to accommodate television, the race started an hour later than last year. However, the sun darted in and out of clouds for most of the race, tempering that problem.

The men’s field stayed tightly bunched as the course unfolded over hills that characterize the first seven miles.

Plaatjes pulled away while going up the last hill at seven miles. He gained the lead less through his own acceleration than through the field slowing on the hills.

From that point on, Plaatjes churned out the miles, laughing to himself when he heard a television announcer tell his audience that Plaatjes had made an serious error in taking the lead, and that he would never hold it.

Daniel Boltz of Switzerland made a late charge but couldn’t catch up, finishing second in 2:11:10. Eddy Hellebuyck of Belgium was third in 2:14:14. John Campbell of New Zealand was fourth and first in the men’s master’s division in 2:14:33.

Marcia Narloch of Brazil was second to O’Brien in the women’s race with 2:32:42, and Olga Markova of the Soviet Union was third in 2:33:27. Priscilla Welch was 11th overall and first in the women’s master’s division in 2:40:20.

Plaatjes was jubilant at the finish line, but moments later he was overcome with emotion.

The happiness came as he reflected on his three previous tries here: in 1988 he was nipped at the tape and finished third; in 1989 his shoes filled with blood from blisters and he dropped out; last year he mis-timed his training and finished fourth.

The emotion that overwhelmed Plaatjes was personal.

Only 10 days ago Plaatjes had no intention of running here. He was responsible for putting together the elite field and did so thinking he would not be in it.

“A lot of people saw it as a conflict of interest,” he said. “I tried to recruit the best runners in the world. We simply can’t compete with the Japanese and the John Hancock Company.”

The Japanese run three lucrative marathons, and the John Hancock Company handles many of the world’s top distance runners.

The elite fields in the first two Los Angeles Marathons were organized by Rod Dixon, who also ran in the race. Dixon has never won in Los Angeles.

Plaatjes decided to run here after receiving a phone call from his brother, Ralph, in South Africa. Ralph said he had been diagnosed with a form of soft-tissue cancer and had six months to live. Ralph, who is 20 years older than Mark, had raised his younger brother.

Plaatjes said decided to run in Los Angeles to earn the money to bring his brother and sister-in-law for a visit to the United States.

Ralph called Sunday morning to wish his brother well. “He said that if I didn’t win, he was going to throw a brick,” Plaatjes said, laughing through tears.

He explained that Ralph used to ride his bike as Plaatjes went on training runs in South Africa. If Mark went too slowly, Ralph would toss a brick at his back.

Plaatjes was less emotional when discussing his eligibility status. He will become a U.S. citizen in 1993, one year after the Barcelona Olympics. He is unsure if he is eligible to represent the U.S. in non-Olympic competition. No one is clear about this, including U.S. track and field officials.

At the moment Plaatjes is the top American marathoner. His time from Sunday is the third-best performance by an American male in five years, behind Ken Martin, who ran 2:09:38, and Pat Peterson, 2:10:04, both in 1990.

O’Brien’s time, too, makes her one of America’s best hopes in the marathon. The 23-year-old from Durham, N.H., was sixth in the 1984 Olympic Trials at age 16 and made the team in 1988.

However, her development was not as fast as some had hoped. She also was hampered by constant comparisons to fellow New Englander Joan Benoit Samuelson.

They have similar running styles and O’Brien’s tiny body--5-feet, 102 pounds--reminds one of Samuelson’s small frame.

O’Brien is coached by Samuelson’s former coach, Bob Sevene.

Her sub-2:30 time has long been a goal, one she accomplished by relaxing and not putting pressure on herself. That may not last long.

“We’ve just got to let her grow up,” Sevene said. “She’s America’s next hope.”

Plaatjes would like to be America’s next hope, but it doesn’t seem likely.

“It’s pretty ironic that South Africa may be in the Olympics before I am,” he said.

* WEATHER MAN: Daniel Boltz, who finished second in the men’s division, trained in snow to run in the sun. C10

* BY DESIGN: Eleventh-hour changes on his special bike helped Jim Knaub win the wheelchair division. C10