IF YOU'VE BEEN dreaming of bicycling the entire length of the Chesapeake & Ohio Canal, this splendid season of new beginnings is the time to do it. A 184-mile-long National Historical Park, the canal stretches from Georgetown to Cumberland, Md., paralleling the Potomac most of the way. It was started in 1828 and took 22 arduous years to complete, at a cost of $22 million, and provided almost a century of leisurely transportation.

The towpath where mules drew barges loaded with coal, limestone, lumber and whiskey still winds lazily through a tunnel of ancient trees, past 75 locks and across 11 aqueducts. Of the original 56 lockhouses, 26 still stand, silent sentinels of the past. Darkey's Lock (Lock 67) was once a famous landmark up and down the canal. It seems that lock-keeper Darkey had seven friendly, red-haired daughters. At Lock 51 there is a lover's graffito, chiseled on the front side of the lockhouse, declaring "D+R 1843."

Stonewall Jackson tried unsuccessfully to destroy the dams that supplied the water for the canal, and Robert E. Lee, after Gettysburg, came close to surrendering when he was forced to remain in Williamsport because the Potomac was flooded. John Brown holed up in a farmhouse near Harpers Ferry, and the Killiansburg Caves along the towpath were a place of refuge for women and children during the battle of Antietam. From the Antietam aqueduct you can see the little village of Antietam Furnace, where workers made cannonballs for the Revolutionary army and parts for James Rumsey's steamboat.

The ride takes you through a land of visual delights, where the redbud and dogwood trees cast pale reflections in the canal's waters; where carpets of Spring Beauty spread out beside the path; where bluebells nod and purple violets and trout lilies and blue phlox work their spell. Ducks glide along the quiet waters and occasionally a fat brown woodchuck or a young cottontail scurries along the path ahead of you.

And there are birds by the hundreds, Some, like the eastern meadowlark, the Carolina wren, the white-breasted nuthatch, the cardinal, the mockingbird, the bluejay and assorted sparrow varieties, stay the year round. But as the April sun begins to warm the earth, the twitting, chirping and songs increase, and soon the scarlet tanager is flashing from bush to tree and the redwinged blackbird, the indigo bunting, the thrush and warbler sing out.

START WITH YOUR bicycle in tiptop condition. It can be a 3-speed, f-speed, or 10-speed. There are no hills on the towpath, so there will be no hard pumping. It's good to have an odometer mounted on your front wheel to watch as it eats up those miles. Carry a tool kit that includes open-end wrenches, a plain screwdriver, a Phillips head screwdriver, a pressure gauge and a greaseless lubricant, as well as two spare inner tubes, a patching kit and a pump.There's an instant tire-repair gadget available now called Pump and Patch. Simply attach the aerosol can to the valve and the hole is sealed and the tire inflated. This is a stopgap, good only for small holes. Include also first-aid items and a snakebite kit. There aren't many poisonous snakes on the towpath, but there are a few copperheads. (In 12 years of riding the towpath I have seen only two and they were lazily draped around a piece of old lockgate, minding their own business.) You may meet a few mosquitos, so carry an insect repellant.

The small amount of clothing needed, your toilet articles and a lantern or flashlight can be carried in saddle bags mounted on the back wheel of the bike. Be sure to include a poncho, sweater, a change of shoes and some Wash'n Dry packets. A day pack is handy for amall items.

Cooking equipment should include a GI-type messkit, a small pan, eating utensils, a small bottle of detergent for dishwashing, paper towels, plastic bags for storage, a canteen for water or Gatorade (a super thirstquencher) and matches. There are minibutane stoves available, which fold up to the size of a healthy hamburger, in case you don't want to build a fire each night.

AS FOR FOOD; keep it simple. Fig Newtons with coffee or tea makes a quick, filling and nutritious breakfast. If you hate Fig Newtons, whip up a batch of powdered eggs, garnished with bacon bits. Carry along a lunch, such as sausage, cheese and crackers, for the first day. After that you can pick up what you need at little grocery stores and snack stands along the way. You can do this for dinner, too, or carry dinner packs of freeze-dried food from a camping supply store. These can be quite good when prepared with a flourish and a well-place dash of dillweed or a soupcon of sweet basil. They include everything from chicken noodle soup to beef stronganoff to potatoes O'Brien. There's even blueberry cobbler for dessert.

Roll you gear in your sleeping bag. Secure it and your tent on the rack behind your bike seat and don't be in a hurry. Allow sid days for your jaunt - which means covering about 30 miles a day. Mileposts all along the way will help you check your odometer:

GET A FRIEND to take you a Cumberland by car. A good place to start is near mile 182, where Wiley's Ford Bridge crosses the canal. (Very little is left of the last two miles of the canal at that end. In fact, there's no canal visible at all, and the towpath dwindles off into open fields.)

To reach Wiley's Ford Bridge take Route 40 to Cumberland and turn off a Route 51, which is Industrial Boulevard. After about four miles, turn right at the first stoplight and go about a mile. Look for the C&O Canal sign near Wiley's Ford Bridge.

Hiker-biker Overnight (HBO) campsites are spaced at roughly five-mile intervals all the way from Cumberland to Seneca at Mile 23. These are on a first-come, first-served basis and have pumps, toilets, picnic tables and fire grills, with firewood close by or for the gatheing. Occasionally you will find the pump-handle missing - that means the water's contaminated. Where pump handles are intact, fill your canteen, in case the next HBO has a bad pump.

If you choose to travel about 30 miles a day, here are HBOs that will make convenient overnight stopping points: Sorrel Ridge at mile 154.1, White Rock at mile 126.4, Opequon Junction at mile 90.9, Blue Ridge at mile 59.5 and Chisel Branch at mile 30.5. The sixth night you'll be in Georgetown.

Food sources along the route are plentiful. Little Orleans Grocery Store at mile 140.9, just beyond the railroad underpass, has not only groceries, cold drinks and snacks, but a wide, shady porch and a flower garden. To get to Caton's Store in Bigh Pool village, wheel off the towpath at about mile 114 and take Route 56 east for a scant quarter of a mile. Just off the towpath at mile 100 is the old canal town of Williamsport, with larger grocery stores and drugstores. Barron's C&O Canal Museum Shop and Country Store is within sight of the towpath near Snyder's Landing at mile 80. The Barrons are canal buffs and their store is filled with canal memorabilia and artifacts, as well as food supplies. Open only on weekends. Himes Grocery and several other grocery stores and snack-stands parallel the towpath at Sandy Hook, from about mile 62 to 61. Then there is White's Ferry Grocery at mile 35.5 and Swain's Lock Snack Stand at mile 16.

Right now the towpath is in reasonably good condition. Floods have been the chief enemy of the canal since its beginning. It took nearly two years to repair the damages of the Johnstown flood in 1889, and after the rampaging 1924 flood the owners closed the canal's operation permanently. Floods of 1936 and 1942 again battered "the big ditch." In 1972, tropical storm Agnes came roaring through the park, uprooting trees, smashing culverts, washing away lockgates and tearing gigantic capstones from several aqueducts. There were 26 breaks in the towpath, one 250 feet across. Now all the breaks have been repaired and the damaged aqueducts restored and stabilized.

The Catoction Creek Aqueduct (mile 51.5) collapsed in October of 1973. A footbridge built by the Park Service was swept away by heavy rains in the fall of 1976. Until this is reparied, bikers can cross the creek on the nearby railroad bridge and be back on the towpath in minutes.

THe only real detour on the whole canal is at the Paw Paw Tunnel (mile 155.8), where stabilization work is going on. The tunnel, built between 1836 and 1850, is a triumph of the 19th-century builder's art. Atunnel for the canal and towpath was drilled through more than half a mile of the solid rock spur of a mountain, to avoid a six-mile bend in the river. The-walls are made of five to six layers of brick. Take a peek inside beofre you begin the mile-and-a-half detour over the mountain. This route is a mite tough, but scenic, wiht sparkling views of the Potomac below and the blue haze of the West Virginia mountains in the distance.

For the detour, leave the towpath at the upstream tunnel portal and follow Tunnel Hill Trail to the top of the ridge; cross the first well-marked road, go 20 yards and turn right on he second dirt road. Follow it down the mountain to where it joins the towpath.

FOR A THREE-MILE section, starting at about mile 90, you get a change of pace, with some rough riding and possibly some pushing. No canal was dug here, since the river above Dam Four, called "Big Slackwater," was deep enough to float the barges. Much of the towpath here is a narrow shelf cut out of the solid rock wall bordering the river. It's an exciting, rough ride through scenes of rugged beauty.

There's one more brief rocky stretch just below Great Falls Tavern at mile 14. This sectin of the towpath was called "Log Wall": When the canal was built, a layer of logs was laid over the rocks and then covered with earth to make a pathway for the mules. Floods have washed away the old corduroy road down to bedrock.

The few rough places are a challenge. The rest is sheer, undiluted pleasure. The towpath beckons!