When Laura MacKenzie first started walking to work from her Northwest home to her downtown office, she recalls feeling "pretty self-conscious. Here I was striding down Connecticut Avenue, all dressed up, wearing these big, clunky walking shoes."

Seven years and a fitness revolution later, MacKenzie blends right in with the growing legions of pedestrian commuters sporting "power suits" and walking shoes. "Now I see lots of other people carrying backpacks to work," says the 41-year-old executive director of the American Hiking Association. "I've always been a big walker, partly for the exercise and partly because it's the one time I'm guaranteed to be alone, without any phone calls."

In a time of high commuting costs and heightened fitness consciousness, more and more people are combining their exercise and transportation needs by walking to work. More than 5 million Americans -- comprising nearly 2.4 percent of the population -- walk to and from work, according to the most recent U.S. Census report. San Diego has the highest ratio of pedestrian commuters, with 5 percent. New York has the greatest number -- more than 400,000 -- many of whom started walking during the city's transit strike and kept on hoofing when they found it cheaper, healthier and, in some cases, faster than other forms of transportation.

Washington ranks seventh as a "walk-to-work" city, with more than 80,000 pedestrian commuters, comprising 2.6 of the population. While the ratio of Washingtonians who walk to work has decreased slightly, their actual numbers have risen: In 1960 there were 57,769 pedestrian commuters, or 2.9 percent of the population and in 1970, 78,504 pedestrian commuters or 2.7 percent.

Although the reasons people walk to work vary, exercise usually heads the list. "Walking is by all odds the most popular form of exercise in America today," claims V.L. Nicholson of the President's Council on Physical Fitness and Sports. "It is the only sport where the participation rate increases with age. About 35 million adults walk for exercise daily and about 15 million more walk two to three times per week."

Under the right conditions walking is "nearly as effective as running," notes Nicholson, who says some of these new walkers are old joggers who, through injury or disenchantment, now prefer to walk. For walkers interested in speed and competition, "racewalking" is becoming popular, with fleet-footed speedwalkers finishing marathons in four hours (9-minute miles) -- ahead of many recreational joggers.

One indication of walking's newfound popularity is the rapid growth of The American Volkssport Association, which attracts hundreds -- and occasionally thousands -- of walkers to its weekend "folk marches." The group doubled its membership from 66 clubs in 1981 to nearly 140 today.

"Walking is a dandy way to shed weight and reduce the hips," says Bill Gale, author of The Wonderful World of Walking (Delta). Rapid walking burns 300 calories per hour, says Gale, who claims the adult hip width has been increasing at the rate of one inch every generation since the invention of the automobile.

A brisk, daily 45-minute walk is more effective for weight loss than dieting, contends Martin Katahn, director of the Vanderbilt University Weight Management Program. Regular aerobic activity raises the metabolism rate by about 10 percent, he says, so the body burns calories faster.

"Most people 50 or fewer pounds overweight don't eat too much," he says in The 200 Calorie Solution (Norton), "they exercise too little. The body interprets caloric restriction as life-threatening and immediately adjusts itself to conserve fuel, much as a bear hibernates all winter to live from its fat."

For people too busy to exercise, the commute on foot can be ideal. "While walking I sometimes carry a hand-size tape recorder," writes Katahn, in a section headed The Executive Walk. "I can dictate letters, make lecture notes or outline a research strategy for one of our weight-management studies."

Walking to work can actually save time, says veteran Washington walker Craig Storti. "Don't forget how long the bus can take on a wet Friday in July."

Walking to work is "a great time to organize your thoughts before a busy day or wind down after a tough one," says industrial psychologist Robert Sleight, executive director of the Walking Association, an American affiliate of Europe's International Federation of Pedestrians. "You don't have the hassles of parking, and unlike some other forms of transportation you know you'll get there."

The biggest pedestrian safety problem, says Sleight, whose group adocates "walker's rights," is "lack of time when the human has the absolute right of way to cross without cars moving. Walkers are taxpayers who aren't polluting the environment, yet they're treated as second-class citizens to drivers. They've got a right to cross a street without running." (Although federal officials say right-turn-on-red has resulted in a less than 1 percent increase in pedestrian injuries, the Federal Highway Administration contracted recently for a two-year study of ways to increase pedestrian safety at right-turn-on red intersections.)

Despite the hazards posed by automobiles, confirmed walkaholics cite varied lifts to the spirit from traveling on foot.

"It's not unlike being back in a small town where everybody knows everybody," says walker Storti, who labels several regulars on his route "my people. You don't speak at all. At most you smile and nod, but the principle's the same. They're there every morning, in the same spot, you recognize them, they recognize you. And you're a liar if you can't admit feeling better for having seen them."

"I find that walking gives me extra energy in the morning," says AHA's MacKenzie. "If I'm tense it's a way of burning it off constructively. I breathe better and my attitude's better."

Walking 9.9 miles from Bethesda to his Dupont Circle office of the Committee on the Present Danger gives 65-year-old Paul Green plenty of time to write poetry in his head.

Other joys include sunrises -- "I've seen hundreds. They never cease to delight me" -- and directing silent pity at "all those healthy young men" waiting for buses.

When he sees "friends in Oldsmobiles" pass by, "I always think," says Green, " 'You should be out here, too.' " CAPTION: Pictures 1 though 5, Rich's Chevy Chasers, $65; Bass Sheffield, $70; Old Maine Trotters ("Walking Lady"), $44; Rockport with Malaysian Crepe sole, $60, and Bass Yorkshire $49.