By now, Jim Fixx's legs are almost as famous as Betty Grable's were during World War II, but not many people are aware that the legs belong to Fixx.

His legs are on the cover of 455,000 copies of his blockbuster best-seller. "The Complete Book of Running," which has been No. 1 on the nonfiction list for months in the top 10 almost constantly since its October release.

Managers of area bookstores say they can barely keep copies of the book in stock and that it is by far outdistancing in sales the dozen or so other running books published to date!

The success of the book is such that Fixx's editor at Random house in New York doesn't want his name used in the stories about the book, largely because he is deluged with manuscripts for other running books.

Now in its 15th printing, the book's original run was 35,000 copies. "We thought it would eventually sell about 200,000 copies, but that it would take two or three years for that," said the editor, who has taken up running in earnest. "But so far it's sold 373,000."

Was he surprised at the book's reception? No, said the editor, not really. "All you have to do is go out to Rock Creek Park or Central Park at 8 a.m. and there's your answer. There are hundreds of people running and there's a lot to know about running."

But the reception surprised Fixx, a 45-year-old writer and free-lance magazine editor with two other books to his credit. He also has held editorial posts at Horizon, Life and McCall's.

"I'm astonished," said Fixx. "I'm a writer and I was looking for something to do. It occured to me that no one had ever done a book which put a lasso around the whole world of running.

"A lot of people have asked me how I could write a 314-page book on something as simple as putting one foot in front of another."

Fixx began running 10 years ago to strengthen a leg muscle hurt in a tennis game. At the time he weighed almost 221 pounds (he's down to 159 now) and smoked a couple of packs of cigarettes a day.

In the course of his running, he began compiling reams of information that later served as the basis for his encyclopedic book, information ranging from apparel and diets to improving time and distance while dodging neighboorhood dogs.

A few years ago, Fixx said, "I began carrying a little spiral notebooks around for about three to four months and wrote everything I could think of in them.

"This is not a book with a strong idea behind it, like 'Be Your Own Friend,' but I wanted to do a book with some personality. So I spent a year just traveling and searching. Then I spent seven months writing it. I just sat in a room and wrote."

The book was rejected by one publishing firm - which he won't name to save the company embarassment - before Random House signed him 48 hours after seeing a four-page outline.

Fixx is getting "standard royalty" for the book, his editor said, adding there are no plans to issue the $10 hardcover book in paperback.

When first-serial rights were offered to some magazines, Fixx said, "There was practically zero interest." But he eventually got a "nice advance" from the Book of the Month Club.

Eventually the book made the best-seller list.

"It was No. 12, then it got to No. 7 and my 16-year-old son said. 'Don't get your hopes up.'" Fixx said. "Then it made to No. 1. It's still No. 1 and I'm completely surprised."

The fan mail has been pouring in since and recent batch included a letter from a Texas physician, praising the book.

"I never even took a biology course after the eighth grade, but no doctor has questioned anything in the book." said Fixx, who recently got a marriage proposal from a Miami woman when she learned the legs on the book's cover were his.

People associated with the book at Random House and involved in the preparation of a runner's calendar for fall release have gotten the running religion.

"I think I proselytize better at long-range, though," Fixx said. "Of my wife and four kids, only my 16-year-old son runs.