Jim Fixx has lost three toenails, pulled a knee muscle, suffered cramps and popped more blisters than he can remember as a result of the 30,000 miles he has run since 1967. Yet the benefits have been substantial too. He cut out smoking and prelunch martinis, slimmed down to 160 pounds, felt better than he had since college—and now, as if to prove that virtue does not have to be its own reward, Fixx has written the surprise best-seller of the winter on his sport.

The success of Fixx's informative, 314-page The Complete Book of Running has astonished the publishing world. Random House has already sold 155,000 hardcover copies, and paperback and overseas editions are still to come. Even to Fixx, Running seemed "too unsexy" a book to interest the average lethargic American. But, he adds, "I didn't worry a whole lot about what other people would think. The book was a kind of love letter to a sport that means a lot to me." (So far he's cleared more than $200,000 from his labor of love.)

The author of two previous books on games, Fixx says he "sat down and cried" when he realized he had hit the big time. "I started writing when I was 13," he explains. "Now I'm 45 and finally I do a book that people like. That's a long time to wait." For 17 years Fixx was a magazine editor based in Manhattan. A former colleague recalls, "When Jim was at LIFE he was a portly slob—the least likely candidate to be a lean runner." Much of the book's appeal stems from Fixx's tips on diet and running gear and his anecdotal story of the long, painful trip from 220-pound chubbie to marathon competitor.

He has become a kind of guru to novice joggers, inspiring converts with tales like that of the 85-year-old woman who recovered her health and happiness by running. At home in Riverside, Conn., Fixx gets half a dozen calls a day from runners with specific questions about injuries or shoes. One Ohio man called up New Year's Day to thank Jim for saving his life.

Ironically, since Fixx went on the promotion tour treadmill, his own physical condition has slipped. "There is nothing worse for your running than writing a running book." He was so exhausted when he reached the AM Chicago show that he turned to the hostess and blurted on live TV, "I'm sorry. I have to leave. I don't feel well." And he walked off camera. After recovering from nausea, he gamely ran through four other interviews later that day.

Hardly a jock as a teenager ("Jim wouldn't have dreamed of playing football," says his mother), Fixx graduated from Oberlin College (where he met his first wife, Mary, the mother of his four children) in 1957. At 35 he became editor of the then third biggest magazine in the country, McCall's. (Of one of his star contributors, Lynda Bird Johnson Robb, Fixx now admits, "She was a terrible writer—on the level of a not very talented high school senior. But she was very popular with the staff.") About seven years ago his marriage crumbled, and in 1974 he was wed again, this time to a public relations woman, Alice, 34, who tried running but could never summon up much enthusiasm. "My apex was seven miles," she confesses. "Then I stopped, and when I did it felt wonderful."

Fixx, who has run in 20 marathons, still averages 10 miles a day—despite dogs, flying beer cans and rude drivers. ("Hey, buddy, why don't ya get a job?" shouted one motorist.) Fixx's advice, especially to new runners, is refreshingly simple: Take it easy. "The biggest mistake people make is to try and run like they're still in junior high school," he says. "In the beginning you should just shuffle along."