Fairfax County Board Chairman John F. Herrity, immersed in his toughest reelection battle, said yesterday it is "a good possibility" that he will not run for office again even if he wins this year.

In an interview this week, he also offered his first apology to friends and supporters for his repeated driving infractions, which have eroded his support.

"I would seriously consider stepping aside after one more term," said Herrity, 55, who has held the county's top elected office since 1976. "Only Mao Tse-tung could be chairman for life."

It was the first time that Herrity, a Republican who is trailing his longtime antagonist, Supervisor Audrey Moore (D-Annandale), in the polls, has given a clue about his long-range political plans.

The remark caught most of Herrity's adversaries and allies off-guard. Some said they thought it might harm his political chances by fostering the impression of him as a lame duck. Others said it could help him to attract undecided voters who might be sympathetic to giving him one last chance.

Moore said in an interview that she was surprised at Herrity's remark, "because I think he really likes to be chairman."

Asked about her own long-term plans, Moore, 59, said, "I'll worry about what I'm going to do later on . . . . I want to make this a really fine place to live. However long it takes is what I'll do." Moore is giving up her seat on the Board of Supervisors to run for chairman.

Herrity, who owns a small insurance business, said he sees a dual threat to the county's prosperity: an antibusiness mood and the national economy, which he thinks will be plagued by rising interest rates, possibly leading to a recession next year.

Herrity said he sees an "unfinished agenda" of transportation improvements that are planned but not yet built in the 399-square-mile county. "After four years," he said "the fundamentals will be in place . . . . " He mentioned the planned cross-county Springfield bypass, which he has long championed, and the proposed outer beltway, which he advocates, as projects that he would like to move toward completion in what could be his final term, which would end in 1992.

"After that," he said, "hopefully there'll be someone else who'll be capable and able to run the county."

James D. Swinson, Fairfax Republican chief, said he had "never heard" Herrity say this would be his last race. "The possible {effect} of it would be somebody would say the day after he's elected that he's a lame duck," he said. Swinson added he thought there would be no great impact on Herrity's campaign.

One official recalled that Rep. Melvin Price (D-Ill.), the 82-year-old former chairman of the House Armed Services Committee, won a tight reelection to a 22nd term in office last year after he announced he was running for his final term.

According to the official, who asked not to be named, Herrity's remark "is a way of saying to people, 'Rally 'round me and give me one more term . . . . ' "

On the issue of his driving record, which has led him to the brink of losing his license and, according to the polls, caused him political damage, Herrity said: "It's something that I did to a lot of people that had a lot of faith in me and a lot of money on me and here I am whipping down the street at 70 miles per hour . . . . I really feel bad about that . . . . That whole thing has cut me to the quick."