Saying "that despite all pressures, temptations and enticements, I am where I want to be," Fairfax County Supervisor Martha V. Pennino announced yesterday she will not run for the position of board chairman, as almost all her fellow Democrats had expected she would.

Instead, Pennino said she would run for a fourth term as the supervisor representing Centreville district, which includes the town of Vienna, her home base, and the fast-growing new town of Reston.

"If Martha had wanted to run, the race would have been hers," said Emilie Miller chairman of the Fairfax Democratic Party. "No Democrat would have challenged her."

The chairman of the board, Republican John F. Herrity, is expected to announce soon whether he will run for a second term. He said recently that he has to decide whether he can afford to work full time as chairman (which pays $15,000 annually) when he should be saving money for the college education of his five children.

The only Democrat to express interest in the position, besides Mrs. Pennino, has been Del. Richard L. Saslaw (Fairfax). Saslaw also is considering a bid for the state Senate seat being vacated by veteran Democrat Omar L. Hirst, who is retiring. But Miller said Pennino's announcement would trigger more interest.

Pennino, who made her announcement in the supervisor's board room, with her husband, Walter, a writer, sitting in the audience, said:

"I was not certain the chairmanship would make me any more effective or give me any more influence than I now have in setting the course of political events affecting my district or the county or the region."

Another factor, which she cited several weeks ago, before her mind was made up, was the high cost of mounting a countywide race. At that time, she noted that it was only recently that she had paid the last debt from her unsuccessful race in the Democratic congressional primary in 1974.