Laura DeWald was favored over the 1,200 women in the recent Marine Corps Marathon and was in the lead after 19 miles of the 26-mile, 385-yard race. But she could go no further. Her right Achilles' tendon became too sore to continue.

Her running in the race two weeks after having competed in the New York Marathon apparently "wasn't such a great idea," DeWald, of Arlington, said after the race. "I really wanted to do well because the race is so close to home and a lot of my friends came out to watch. And even though I ran in New York, I felt I still had a good one in me."

There'll be more marathons for the 25-year-old DeWald. After only three years of distance road racing, she has become one of the country's top women amateur distance runners, according to Running Times magazine. She has her sights on being one of the three women who will represent the United States at the 1984 Olympics marathon in Los Angeles. It will be the first Olympic marathon to include women.

DeWald holds the national record for women for a 20-mile race: two hours, two minutes and 20 seconds. This year, among American women, she holds the seventh fastest marathon time, 2:34.59, which also is her best time. She plans to cut that time to 2:30. The American record is 2:26.12 by Joan Benoit. DeWald's best time is the 17th best of all times by American women in U.S. marathons.

DeWald's success in races ranging from 10,000 meters to marathon distances persuaded her recently to quit her $22,000-a-year job as a civil engineer for Arlington County. She now runs full time on the Brooks Shoe Inc. racing team, competing in races throughout the world at the expense of Brooks and race promoters.

DeWald's interest in running dates to her days at Washington and Lee High School. In 1975, she was a sprinter and was captain of the school's track team, which won the Great Falls District Outdoor Championship that year. She went on to the University of Virginia, where she continued running on the cross country and track teams.

"I ran my first marathon in 1979 on a whim after I had built up to 30 miles a week," she said. "But before that, I was burnt out . . . during my last year running at UVA and didn't run for six months. But my friends encouraged me to try it (the 1979 Marine Corps Marathon) and the race is close to home. I found it was a lot more exciting to run on the roads than on the track."

DeWald has won the admiration of her former high school coach, Barbara Reinwald. "Laura had a great deal of devotion to running at Washington and Lee that continued at the University of Virginia and today," Reinwald said. "She won the Great Falls district half-mile title in 1975. I'm not surprised she is one of the best road racers in the world."

"Many times I'll see her running though the streets of Arlington on my way home from work," she said.

Occasionally, DeWald runs with local high school runners, but she mostly trains alone. Her runs explore Arlington's streets, the George Washington Parkway, Burke Lake in Fairfax, the C&O Canal towpath near Carderock and the Arlington part of the Four Mile-Run Trail. She is a member of the Washington Running Club, which, she says, includes about 150 members throughout the Washington area.

The demand for training and competing prompted her to quit her job and concentrate on running.

"I refused several chances to run in races abroad because, at the time, my job duties were more important than racing," she said.

While working as a civil engineer, she ran in the early morning and early evening because she worked regular business hours during the day. This meant running often when it was dark.

"The best thing about Laura running full time now is that she can train during the day," said her father, Arthur DeWald, a retired Army officer who works at the Computer Sciences Corp. in Falls Church. "It's not safe to run at night."

DeWald trains consistently, running 70 to 80 miles a week. On weekdays, she takes an easy run around 9 a.m. and longer runs in the afternoon. Three days a week, she uses Nautilus machines to build upper-body strength. Once a week she runs interval training for speed, which involves repeatedly running a distance in a certain time with a fixed amount of rest.

On weekends, if she is not racing, DeWald will take long, less strenuous runs. On Sunday, she runs 16 to 20 miles.

Occasionally, she travels to the hills in Hanover, Pa., to train "more intensely" with her coach, Will Albers, a former Arlington resident.

Albers, a world class marathoner who attended Robinson High School and George Mason University, said the money DeWald can earn from road racing can at least match that she made as a civil engineer.

Depending on her finish and the popularity of the race, DeWald can win $1,000 to $15,000 a race in prize money. In addition, race promoters offer lodging and travel expenses.

But the money is not easily accessible. The Track Athletics Congress, the national governing body for amateur road racing and track and field, requires prize money to be put into a trust fund. The money can be used only for "legitimate" expenses, a TAC official said.

Although DeWald occasionally receives money from races, she is considered an amateur and remains eligible for international competition, including the Olympics, according to TAC guidelines.

Albers said DeWald is "an employe of Brooks on a contractual basis," but would not disclose the specifics of the contract. Brooks gives DeWald running equipment and handles her travel and race details. Moving Comfort Clothes, in Alexandria, also provides DeWald with running equipment.

DeWald has fared well in races throughout the world. In early August, she finished second in the Rio De Janiero Marathon. Two weeks later, DeWald won America's Finest City Half-Marathon in San Diego. But DeWald's fondest memory comes from the Osaka Ladies Marathon in Japan last January. At that race, DeWald finished fourth, but was the first American woman finisher. It also was there that she ran her best time of 2:34.59.

In last month's New York Marathon, DeWald finished 18th out of 1,899 women who finished.

A Japanese television station recently spent a week with DeWald. A 90-minute documentary on top American women runners will air in Japan in mid-January. "They Japanese are very responsive to runners," DeWald said.

Albers said DeWald must develop a killer instinct if she wants to improve. "Laura has the physical ability to be the best in the world," he said. "But she has a low threshold of pain."

"My biggest weakness is my mental attitude," DeWald acknowledged. "Some people call me 'the ever-smiling Laura DeWald' because I smile a lot when I run. I'm probably too comfortable in my races. I realize I've got to get a bit tougher and push myself harder. That's the only way I'll be the best in the world."