John F. Herrity-- jogger, bummer of cigarettes and self-ordained guardian of the Northern Virginia suburbs-- strode purposefully into the studios of WJLA-TV yesterday, grabbed a chair and said, "Boy, am I tired."

The chairman of the Fairfax County Board of Supervisors announced that he'd been up nearly all night Wednesday dealing with Tropical Storm David, that he'd spent a long day with Virginia Gov. John N. Dalton and at other governmental meetings on Thursday, and that his schedule for Friday was extremely full.

Despite claims of fatigue, the bald, plain-spoken Republican didn't look at all tired. He'd shown up for the taping of a television debate with Vivian Watts, his Democratic challenger for reelection in November, and he looked like a well-rested politician confident of victory.

He has legitimate reasons for confidence. Herrity is probably the best known politician in Northern Virginia. When he pulls on his running shoes and jogs to work in the name of energy conservation, the media covers it.

He has a smooth-rolling campaign organization that is plugging into contacts made last fall in his unsuccessful congressional bid. His campaign people report no trouble in raising money to meet campaign expenses.

And Herrity, in an era of massive opposition to governmental spending, has created an image as the archrival of those free-spending liberal pie-in-the-skiers.

To combat all of Herrity's strength, Vivian Watts, a long-time community activist and former president of the League of Women Voters in Fairfax, said yesterday she has no choice but to rely on "issues and substance."

It's on the issues, such as controlled growth for booming Fairfax County and fair pay for county employes, that Herrity is weakest, says Watts, 39. She says that Herrity decides what issues are important simply by picking up on the uninformed fears of taxpayers.

"Herrity feels the wind on the issues and runs with it politically. He closes his mind and ears to other points of view. It becomes the world according to Herrity," says Watts.

In the taping of the debate yesterday (to be aired on Channel 7 at 7 p.m. Sunday), Watts and Herrity clashed over how high pay raises should be for county teachers and other employes.

Watts said that the Board of Supervisors' decision last spring to limit pay raises in the coming year to 5.5 percent is unfair in light of President Carter's recent recommendation that federal employes get a 7 percent raise.

Herrity, who was the object of considerable enmity from Fairfax teachers this spring because he insisted on keeping salary increases low, told Watts he thought it was wrong for her to pledge taxpayer's money in return for the political support of county employes.

Watts denied she had done such a thing.

After the debate, Watts admitted that there are not many votes to be gained by supporting county employes in their effort to get more money. Each 1/2 percent increase in salary for the county's approximately 20,000 employes costs taxpayers an additional $1.4 million.

But Watts says county employes deserve more money and she is going to make it an issue even if it doesn't attract a lot of voters.

Herrity, after the debate, said he can't understand why Watts is interested in an issue that doesn't interest the broad spectrum of voters in the county.

While Watts is struggling to interest Fairfax voters in issues, she says her campaign is not having an easy time attracting financial contributions. She said the campaign goal is $30,000 and that so far she's managed to raise only $10,000.

The money-raising problem in a countywide race kept Herrity's strongest potential challenger, Supervisor Martha V. Pennino (D-Centreville), from running. And Watts said yesterday her lack of money is keeping her from planning a major mailing to the county's 120,000 registered voters.

Herrity, will only say that his goal is to raise between $35,000 and $50,000 for the campaign. He said it would be a "tactical" mistake to tell anyone of this point how much he's already raised.