* AHS Fourth Annual Hike-In: Oct. 8 to 11 at Prince William Forest Park in Virginia, sponsored by the American Hiking Society. Workshops include nature photography, edible wild foods, orienteering, solo trekking and backcountry medicine. $35 (nonmember) fee for four days includes cabin or campsite. 234-4609.

* Easter Seal Walk-a-Thon: Sixth Annual fund-raiser Oct. 9, sponsored by WKYS. Walkers may do all or part of the 20-mile course through the city, beginning and ending at the Lincoln Memorial. Registration 8 a.m. at the memorial; walk begins at 9. For sponsorship call 232-0037.

* Detailed listing of walks runs weekly on the "Sporting Life" page of Friday's Weekend section. Walkers United

* Walking Association: Pro-walking advocates affiliated with the International Federation of Pedestrians publishes a newsletter concerned with pedestrian safety and walking for health. Membership $12 year. Write Walking Association, 4113 Lee Hwy., Arlington, Va. 22207.

* American Volkssport Association: Sponsors public "folk marches" through 136 clubs nationwide. For information write AVA National Headquarters, 1001 Pat Booker Rd., Suite 203, Universal City, Tex. 78148.

* Washington D.C. Volksmarch Club: Offers 6- and 12-mile marches most weekends -- including one at Prince William Forrest Park Saturday and Sunday -- with a $4 fee for those interested in a medal upon completion. Call Chuck Burrows, 521-3662, after 5:30 p.m.

* Air Force Data Services Center Volkssports Club: Runs 6- and 12-mile walks throughout the fall, open to the public. Call Selina Parker, 697-4264.

* Bird Walks: Sponsored by the Audubon Naturalist Society at locations such as Rock Creek Park, Sandy Point and Pennyfield-Violets Lock on the C & O Canal, free. 652-9188.

* Sierra Club: Activities listed on Dial-A-Hike, 547-2326.

* Wanderbirds: Sponsors Sunday walks at parksites within 100 miles of Washington, via chartered bus, for a cost of about $9 (50 cents extra for nonmembers). Slow, moderate and fast walks scheduled at distances ranging from 4 to 14 miles, with fall and spring marathon hikes. For information, send a self-addressed, stamped envelope to Mary Shaughnessy, 200 Locust St. SE, No. 407, Vienna, Va. 22180. Walk Talk

* Walking for Exercise and Pleasure: Free booklet available by writing The President's Council on Physical Fitness and Sports, 450 Fifth St. NW, Room 7103, Washington, D.C. 20001.