Fitness for the 35 and Over Set will be taught at George Mason University this fall, beginning Aug. 29. The program will be led by Dr. Fred Schack, a fitness specialist experienced in working with older adults. The 10-week course is designed for any individual, regardless of physical condition or experience. It will run from 5 to 6:30 p.m. Aug. 29 to Nov. 4 on Mondays and Wednesdays in the auxiliary gym of the Physical Education Building (not the Field House) at George Mason University, 4400 University Dr., Fairfax. The cost for the entire session is $42.50 per person. To register for the classes, call 323-2828. Callers unable to reach the university to register can show up at the times listed above.

The Arlington Jaycees, a leadership training organization for people ages 21 to 39, will seek business community sponsors at the Arlington County Fair, tonight through Sunday.

The fair will be held at the Thomas Jefferson Community Center, 3501 S. Second St., Arlington. Jaycee booths will be open from 7 to 10 tonight, and from 10 a.m. to 9 p.m. Friday and Saturday, and from noon to 5 p.m. Sunday. All proceeds from the booth games will go to the Cystic Fibrosis Foundation, the Muscular Dystrophy Association and Camp Virginia Jaycee, a summer camp for retarded children. Anyone wishing to join the Jaycees or become a corporate sponsor can call 369-7311 or write to The Arlington Jaycees, P.O. Box 1281, Arlington, Va. 22210.

The American Lung Association of Northern Virginia will hold Stop Smoking Clinics from 7 to 9 p.m. Sept. 8, 10, 14 and 17 at Calvary Hill Baptist Church, 9301 Little River Turnpike, Fairfax. The clinic features films, discussion and instruction in relaxation and self-control. For a $20 fee, participants can discuss difficulties and use group support to keeptheir commitment to quitting. For more information, call the association's office at 703-591-4131. Preregistration deadline is Sept. 3.

An adult coed volleyball league will be organized by the Falls Church Recreation and Parks Department. Matches will begin in September, and are open to everyone age 16 and older. Registration will be held until Sept. 1. For further information, call 241-5003.