A guidebook to trails being built by the Northern Virginia Regional Park Authority provides detailed maps of more than 66 miles of hiking, biking and horseback riding trails from downtown Arlington to rural sections of Fairfax and Loudoun counties.

The 24-page booklet called "Happy Trails," is "one response to the eight public hearings which the park authority held last winter," said park authority executive director Darrell Winslow. The overwhelming need in Northern Virginia, expressed at the hearings, was for trails, particularly biking and horseback riding trails, Winslow said.

The booklet, which sells for $1, includes two portions of what will be the regional park system's two longest and most scenic trails, Bull-Run Occoquan and the Washington and Old Dominion Railroad trails.

Along the 25-miles shoreline of Bull Run and Occoquan Reservoir there are already some 16 miles of continuous hiking and horseback riding trails, with the last nine miles to Sandy Point just above the town of Occoquan expected to be completed next year.

The 42-mile W&OD railroad right-of-way, which stretches from Arlington to Purcellville in the foothills of the 'Blue Ridge Mountains, is expected to become one of the most popular and scenic bike trails in the East when it is completed in the next few years. Only a short 1.5-mile stretch in Falls Church is now open but by next spring bicyclists should be able to travel from Arlington to Vienna on a wide, paved bike trail that will include bridges over the Beltway and I-66, both now under construction.

Other paths mapped and described in the pamphlet include horseback and hiking trails around Pohick Bay Regional Park, hiking trails at Arlington's Potomac Overlook and recently opened Upton Hill Regional Parks and hiking trails at Loudoun County's Algonkian and recently opened Red Rock Regional Parks.

The booklets may be purchased at any regional parks or by writing to park authority headquarters, 11001 Pope's Head Rd., Fairfax, Va., 22030, and including 25 cents for postage.