YEAR IN and year out, having fun can be a big hassle. For every hour spent on the boat, on the slopes, on the trail or in the woods, it seems there's always four hours' worth of cleaning, waxing, repairing and otherwise making last week's fun leftovers ready for next week's fun.

The leisure life in itself can be a lot of trouble. And if you get caught up in an organized activity, especially one in which some government agency is involved, well, the fun is often hardly worth the effort. I remember my sense of shock a quarter- century ago when I heard that the park bureaucracy had people standing on line to see over the south rim of the Grand Canyon. Patiently standing, that is.

So it was with a certain sense of wonder that I watched last year's annual Bay Bridge Day (the 1Oth one) come off without a hitch, when 50,000 souls turned out for a glorious walk the length of the eastbound span of the William Preston Lane Jr. Memorial Bridge across Chesapeake Bay. And had fun doing it.

What was wonderful about this event was that all these people were not allowed to park their cars at either end of the bridge. Instead, they were directed to parking facilities miles from the Bay, and then were bused smoothly and quickly and with a minimum of down time from parking lots to bridge and back.

This year's Bay Bridge walk is scheduled for Sunday from 9 to 4, unless heavy rain or high winds force cancellation, and there's every reason to believe that the Maryland Transportation Authority will have everything running just as smoothly as it did last year.

The drill:

You park, then get on a bus, then walk, then get on a bus, then drive away. As simple and untrammeled a way to have fun as anyone could imagine.

The whole thing is free, including the bus rides from designated parking areas. The bus stops are clearly marked and the buses come along at close intervals. The walk is 4.3 miles, because that's how long the bridge is. The total elapsed time last year for one group was just a minute or two under two hours.

All walkers begin from the Eastern Shore side of the bay and walk west toward the toll plaza area, where buses load for the parking lots.

This walk takes you 185 feet up and past a number of signs indicating points of interest, directing your attention to ship channels below, buildings on the horizon and a fascinating overview of the hydraulics of the Bay, where several rivers, including the Choptank, have their termini. Other interesting spots (with signs directing you to them) include the Annapolis anchorage for ships waiting to get in to Baltimore, the Maryland Capitol dome, the Naval Academy grounds, the Sandy Point and Thomas Point lighthouses, and the remains of the ferry slip that used to serve Bay-crossers before the first span of the bridge was opened in 1952.

If the folks in charge of this event make it easy and pleasant to get to and from the bridge, they also make the walk comfortable and safe. There are chemical toilets along the way, and tanks with drinking water dispensed by members of the Maryland National Guard. All sorts of other help is available from volunteers (who wear bright orange caps) and officers from the Toll Facilities Administration police.

No pets, bicycles, skateboards or food are allowed on the bridge, nor is jogging or running. But radios are allowed, and if you don't like your music loud and blaring you'll have to find a nice rocking-chair saddle between clusters of walkers. This is not an impossible task: There's never a sense of overcrowding, and there's always a quiet eddy among the walkers where the less-than-fervent hiker can stand, or sit, and enjoy the sun and water and sky.

As a matter of fact, one of the delights of this bridge walk is that there's no real destination: There simply is no reason to get from one end of the bridge to the other other than the walk itself. As far as the eye can see below, water stretches off toward the sea. Destinations are for the ships in the channel below and you can stop a moment or two to dream about where those ships have been, and where they'll be in a week, after loading or unloading in Baltimore.

And special rewards await those who keep an eye out for the silver-tipped wings of the marine birds on and over the Bay and the bridge: the gulls and the great blue herons that cross the marshes inside the shoreline, the pelicans and the ever-so-occasional osprey, the trio of ducks flying like manic bats and the Canada geese who forgot, several seasons ago, to return to the Arctic Circle.

Last year's walkers were all smiles, the sky was sunny and the breezes brisk. All you need to do to prepare for this bit of fun is to dress appropriately. Recommended clothing: layers, to put on when the Bay wind picks up and the shadows lengthen, and to strip off when the sun streams down and the grade is uphill. Good sturdy shoes, of course, and socks.

And after the walk, why not sample the fare at one of those U.S. 50 restaurants around Annapolis you've always seen in passing but never had time to visit before? You've earned it.

ELEVENTH ANNUAL BAY BRIDGE DAY WALK -- Sunday 9 to 4; last chance to get on the bridge, however, is 2 p.m., with the last buses leaving the lots for the bridge at 1. The parking sites are Navy-Marine Corps Stadium, Annapolis; Anne Arundel Community College, College Parkway off Maryland Route 2 (Ritchie Highway) in Arnold; Kent Island shopping center on U.S. 50. The bridge people suggest that hikers from the D.C. area use the Navy stadium parking lot. To get from the Beltway, take U.S. 50 east to the Rowe Boulevard exit in Annapolis; continue on Rowe for about 11/2 miles to the stadium, which fronts on Rowe. Road signs and arrows are to be posted on U.S. 50 and Rowe Boulevard the morning of the walk. No charge for walk or bus or parking. For information, call 301/563-7182. Cancellation report: 301/355-2080 (morning of walk).