Autumn is the best of times to use the more than 65 miles of hiking trails operated by the Northern Virginia Regional Park Authority.

After the cicadas have finished filling the air with nature's own Muzak, a shrill sound that echoes through the woods in summer-time, hikers can watch the parks ignite in brilliant displays of color.

To view the fall colors on foot, visitors to the Northern Virginia Regional Park Authority's Fountainhead Regional Park along Lake Occoquan can walk any of three trails that wind over heavily wooded hills and ravines to views of the lake and low marshlands.

If the smells and sights aren't dramatic enough for you, you might also want to contemplate the silent revolution creeping across the forest. In the next couple hundred forest, spreading its ever-growing shadow and overtaking the dogwood, poplar, hickory, oak, pine and other trees. Maybe you'll come upon the unusual muscle tree or iron wood. You'll know it by its thick ridges, which look like well-developed muscles. Or slap it with your hand and you'll know how it got its other name.

You might also catch a glimpse of the deer, beaver, turkey, squirrel, raccoon or 200 species of birds who make the park their home. If you're lucky, a bald ealge could soar overhead.

A two-mile hiking trail rambles through the hilly terrain set back from Lake Occoquan, and a booklet describing things you'll see along the way is available at the park's visitors' center. The park also has a one-mile hiking trail that follows a high ridge overlooking a small cove and joins with several short spur trails to scenic overlooks. Neither trail is difficult to hike, but they do require stamina. The second half of the two-mile trail is almost entirely uphill.

Rugged types might want to back-pack along the 2 1/2-miles hiking trail, which traverses steep slopes of hemlock groves and rhododendron underbrush. The trail is well marked with log siding and eventually joins an unpaved roadway. Overnight camping is permitted in a primitive camping area beside a stream. Campers must hike there, carrying all their firewood and supplies with them. No buildings or modern facilities await. Reservations for the primitive camp site may be made in advance.

The official fall colors day at Fountainhead is this Sunday, and chief naturalist Earl Hodnett will lead a free guided hike at 2 o'clock, beginning at the park's main parking lot. If hiking isn't your idea of fun, Hodnett will conduct two fall color cruises that same day on Lake Occoquan. These begin at 10 and 11:30 from the visitors' center. Reservations are required for the cruises, which cost $2 for people 12 and older, and $1 for those under 12. Call 278-8880.

Unguided pontoon cruises will also begin at 3 o'clock Sunday and 1 p.m. the following two Sundays, Oct. 22 and 29. These cost 75 cents per person and no reservations are necessary.

Also available at this park are fishing, boat launch ramps, a visitors' center with tackle and bait, sandwiches, snacks, coffee and soft drinks and rowboat rental for $3.50 a day, picnic tables and grills, cruises on a sightseeing boat and an observation deck with a spectacular view at the widest point of Lake Occoquan. Fountainhead is about 25 miles from Washington, at 10875 Hampton Road, Fairfax. Call 250-9124 for directions.

Hodnett will also lead a free fall colors hike at Potomac Overlook Regional Park in Arlington Sunday, Oct. 22, beginning at 1 p.m. Reservations are suggested, and can be made by calling 528-5406.

Other trails operated by the Northern Virginia Regional Park Authority are:

ALGONKIAN PARK - The one-mile looped hiking trail meanders through the seasonally swampy woods. It will be rerouted later this year, when construction is to begin on the park's expanded recreational facilities. Hikers can use the current trail through October. It's a good place for birdwatchers, who report sightings of infrequently seen species. Birders can also follow both the high and low banks of the Potomac shore along a mile-long dirt path. All the trails are easy to hike. Algonkian is about 14 miles from Tysons Corner, just over the Fairfax County line in Loudoun County. Call 450-4655 for information.

BULL RUN PARK - One of the spring's best wildflower shows can be see from the one-mile and 1.8-mile hiking trails in this thousand-acre park of woods and meadows. The trails are easy to hike, and since they run through some swampy areas they're a good place to see a variety of birds. The park also has a five-mile trail for horseback riding. Bull Run is about 30 miles from Washington, at 7700 Bull Run Dr., Centreville. Call 631-0550 for information.

BULL RUN-OCCOQUAN TRAIL - There's a 14-mile hiking and horseback riding trail from Bull Run to Bull Run Marina. An additional nine-mile stretch is planned for future development from Bull Run Marina to Sandy Point. Traveling downstream from Bull Run Park, the hiker can find beautiful scenery beyond Ordway Road. The stream valley gradually changes to low, sharp hills with hemlock groves, rock outcroppings and stream "islands." The best section for such sights is between Hemlock Overlook and Bull Run Marina, but that section of the trail is presently under improvement to accommodate both pedestrian and equestrian uses. It is expected to be serviceable in 1979.

POHICK BAY PARK - Walking beside the water, hikers here may sight a bald eagle. The park occupies a spectacular bayside setting on the historic Mason Neck peninsula of southern Fairfax County, 25 miles south of Washington. It's an area where deer and beaver roam and osprey and egret wheel overhead. There are three major hiking trails here, 1.6, 0.8 and 1.4 miles long. They are moderate-to-slightly strenuous to walk, and cover many different habitats. There's also an equestrian trail, which runs 3.3 miles within the park and continues along Pohick Creek for an additional five miles. The park is located at 10651 Gunston Road, Lorton. Call 339-6100 or 339-6104.

POTOMAC OVERLOOK PARK - In the heart of the hustle and bustle of urban Arlington, there lies a tranquil nature sanctuary where hikers can see tulip poplars and oaks that are hundreds of years old, moss and ferns clinging to stream banks and the old spring that was a source of water for the Necostin Indians who once inhabited the slopes. The park, at 2845 Marcey Road, Arlington, has a two-mile trail for hiking, including a quarter-mile interpretive trail with numbered posts along the way. A coordinating booklet available at the park's nature center describes things you'll see at each post. Connections at both ends of this trail will take the hiker to Arlington County's Donaldson Run trail. The trail is easy-to-moderate to hike. A 0.2-mile stretch of Marcey Road, which runs through the center of park, is used for bicycling. Call 528-5406 for information.

RED ROCK OVERLOOK - There is a three-mile hiking trail system here, with some slightly strenuous spots in hilly areas. The secluded trail system is designed so that the subloop connector trails follow ridges and converge around the confluence of several small streamlets. The overlook points are situated atop high, sheer cliffs with panoramic views toward the bend of the Potomac River and the distant Shenandoah foothills. The hiker will also find a variety of wildflowers here, and it's not uncommon to come across deer along the way. The park is about 33 miles from Washington, near Leesburg on Edwards Ferry Road in Loudoun County. Call 278-8880 for information.

UPTON HILL PARK - The one-mile hiking trail system in this urban nature sanctuary is fairly flat and easy to walk. The 26-acre part is astride the Arlington-Fairfax County line, at 6060 Wilson Blvd, Arlington. It is open to the public on a limited basis until an access road and parking facilities are completed in 1979. At one point along the trails, several streamlets converge, providing an interesting focal point of fern growth. Call 241-1410 for information.