Steve Jones, a 29-year-old Royal Air Force corporal, set a world record to win America's Marathon-Chicago today. It was his second attempt at running a marathon and the first time he has completed one.

Jones, timed in 2 hours 8 minutes 5 seconds, ran away from one of the toughest fields assembled in recent years, opening a firm lead in the 20th mile of the grueling 26.2-mile race that was never challenged. He cut eight seconds from the previous world record of 2:08:13, set by Alberto Salazar in the 1981 New York Marathon.

Portugal's Rosa Mota, the 1983 winner here, today repeated her triumph, taking the women's division in 2:26:01.

Jones went ahead between the 19- and 20-mile marks with a a 4:47 mile. He followed that with a 4:46, a 4:43 and a 4:47, actually going faster at the end of the race than he had at the beginning.

Jones, a jet-fighter mechanic with a reputation as an especially tough-minded competitor despite inexperience at marathon distances, defeated Carlos Lopes of Portugal, the 1984 Olympic gold medalist, who took second; and Australian Rob de Castella, the world's top-ranked marathoner last year, who placed third. Lopes' time was 2:09:06 and de Castella's was 2:09:09. Salazar did not compete.

The race began at 8:45 a.m. (CDT) in rain and buffeting winds off Lake Michigan. But by the 11th mile, the weather had improved, settling into almost ideal running conditions: light breezes, no rain, temperatures in the upper 40s. A record 10,114 runners, most of them amateurs, competed. The inclement weather at the start kept the spectator crowd down, but contributed to fast going in the first third of the race that put a world record within reach.

Jones said later he was shocked at the early pace and once asked de Castella if the elapsed times they were seeing could be right.

Jones' feat earned him $35,000 and also thrust the Chicago race into the forefront of world marathoning. This was only the eighth running of the event here, but the record time reflects the organizing abilities of race director Bob Bright, who spent months lining up runners, and the sheer power of money. The Beatrice Companies, a Chicago-based conglomerate, has put up about $3 million to stage the marathon, including a world record jackpot for winners and top finishers.

Jones, said he was "very surprised" that Lopes and de Castella, two accomplished marathoners, "just didn't come" when he first took the lead in the 18th mile.

He dueled with fellow countryman Geoff Smith, the 1984 Boston Marathon winner, for a few minutes. Then he held off a brief challenge from Gabriel Kamau, a Kenyan who had fallen in the 18th mile but recovered and forced himself back among the six leaders.

Jones said that once he found himself alone and ahead by about 100 yards, "I felt in danger . . . I didn't want them to catch me."

He had nothing to fear. Lopes, speaking through a translator, said, "When I saw Jones running alone, I did not feel strong enough to go with him." He decided to stay in second "and see what happened."

What happened was that Jones continued to lengthen his lead. "All I wanted to do was to keep the guys behind me." His masterful performance completed a thrilling turnaround for a little-known runner whose career not long ago had been marked by trouble and disappointment, "a pretty bad year" until today.

A Welshman who dropped out of school at 15 and worked at odd jobs before enlisting in the RAF 10 years ago, Jones has run most of his life, but does not consider himself a marathoner. He prefers 10,000 meters and trained hard for that race for the 1984 Olympics.

He arrived in Los Angeles expecting to do well, but came in eighth. Then, he and his longtime coach parted ways and Jones was unexpectedly on his own. But one goal loomed above all others. In 1983, competing here for the first time, he had pulled a muscle the night before the race, stepped into a pothole with nine miles to go and was forced to withdraw.

Instead of returning to England, Jones continued using his RAF leave and a bonus vacation he received for re-enlisting for 22 more years' military service. ("I like the work, and it's a secure wage, you know.")

He trained on his own at a Utah ski resort, won the Dayton Half-Marathon Sept. 23, won an eight-mile race in San Francisco a week later and placed second to Lopes this month in a 15,000-meter race in El Paso, Tex. He raised his weekly running totals to 95 miles from 75 miles, and waited for Chicago.

"I really wanted to finish the race because of what happened here last year," he recalled. He got up at 5 a.m. today, downed some muffins and jam, and bid his wife, Annette, goodbye.

Jones also kept in mind the advice of Britain's 1984 Olympic bronze medal marathoner, Charles Spedding, not to challenge the leaders until the 20th mile. "Don't go too soon, " Jones said he told himself.

When he finally let go, he had the field to himself. Heading down the final 150 yards he could see the huge digital clock at the finish line ticking off the seconds. He knew the world record was within his grasp. He tried to pick up the pace, but felt his thighs tightening, his energy dying.

"I thought the clock was running faster than I." But Jones bore down to take the race and record.

Now, he will have some money in the bank, with many lucrative contracts about to head his way. But Jones hasn't faced that yet. "I'm just a runner. I like to put the shoes on and go."