Seven months ago John Herrity, Chairman of the Fairfax County Board of Supervisor was in the hospital for his second serious heart attack in a possible times. This Saturday he will be in a four-mile cross country race.

Herrity is able to enter this year's race, to be held in Vienna, due to a regular exercise routine that has increased not only his physical endurances but his confidence as well, he says.

His exercise program began on the civics of this cardiologist, Dr. Robert G. Metthews, while Herrity was still re-operating in the hospital. He began by walking within the hospital several times a day. When he returned home, he took morning walks, first around the block and then longer distances until he could walk a mile with ease. Jogging followed in the same pattern.

Herrity now routinely runs three to four miles every morning around his Springfield neighborhood. He feels "great," he says, and last week he played two sets of tennis.

Although the fear of another heart attack diminishes with time and exercise, Herrity says, "You never completely forget about it. "It's there in the back for your head - in a gray area comeplace."

Overcoming the psychological barster, the fear that overexertion will bring on another heart attack, is at least as important as developing physical endurance.

"The mental thing is very hard to conquer," he says. "What a person has to do is decide he is going to live a full life - that he is going to function - and do the thing that is required."

It has not been easy for Herrity. He has twice been to the emergency room on false alarms. His palse turned out to be fatigue or overeating as by the exercise.

Herrity attended an exercise class at George Mason University, recommended by his doctor and run by Dr. James Metcalf for former cardiac patients. The class meets once a week and is designed to build heart muscle strength.

On the whole, Herrity reports that his doctor is very pleased with the results.It helps that he has shed 30 pounds, modified his diet a little and almost, but not entirely, given up smoking.

This year's race, sponsored jointly by the county recreation department and park authority, has three events, of which the cross country is the last. The races will be at Nottaway Park, 9601 Courthouse Rd., Vienna, and are open to Fairfax County residents.

The first race, to be at 10 a.m., is a one-mile run for youngsters under 12 years of age.

At 10:30 a.m. a two-mile run for women and girls will be held.

The four-mile cross country is open to any county resident and will begin at 11 a.m.

Persons running in the races who are not already registered should arrive early to do so. A parent's signature is required for anyone under 18.

Larry Fones of the Recreation Department said 80 persons participated in the race last year but more are expected this Saturday. Medallions will be awarded to the winners of all three races.