The Complete Capital Runner's Guide. By Kasia Johnson and Kim Diffendal. $6.95.

Washington is undisputably one of the best cities in the nation to be athletic. There are bicycle trails, peddle boats, volleyball courts, sailing marinas, and places to run -- lots of places to run.

There are so many running routes and local running activities that it is difficult to know where all of them are. That's the dilemma in which Kasia Johnson, 27, and Kim Diffendal, 29, found themselves when the two sisters moved back to the District from Saudi Arabia two years ago.

When they couldn't find accurate, detailed information about the running scene here, Johnson and Diffendal decided to compile a smorgasbord of running facts and figures including extensive maps and descriptions of areas in which to run.

They found that "there is no reason to run in the same place all the time," Johnson said in an interview. "That really opened up our eyes."

This 99-page paperback is very informative, but the authors jump right into a heavy discussion of "running in unfamiliar areas" without setting up the book through an introduction. Some background information on the history of running in Washington -- including trivia on the famous runners from the area, some of the traditional running races in the city and discussion on how the running movement has progressed here -- would have given the reader some perspective.

Other than this oversight, the authors take the reader on a well-organized route with discussions of safety, the District's weather and sunrise and sunset times. Then Diffendal, a professional cartographer, maps 19 routes through the metropolitan area, indicating mileages, water fountains, restrooms, Metro stops and parking facilities. Along with the maps are detailed descriptions of the surfaces and terrain, and some pertinent tips about each area.

"The bike trail network is tremendous," Johnson said. "It worked out really well for the bikers, and runners can use the bikers' trails, too." But she said that running on the Mall in the evening, which was her route when she started running in May 1983, is "one of the prettiest places to run with the sun setting behind the Lincoln Memorial."

The book, which the authors published themselves, is available in some local running stores. The You Can Do It! Kids Diet By Dee Matthews with Allan Zullo and Bruce Nash. Holt, Rinehart and Winston, $14.95.

In an era when yuppies dictate the ins and outs of style, the appearance of a diet book for yuppie youngsters probably was inevitable. Now we have a guide which, the foreword advises, can help overweight children "cope with temptation."

Clearly, however, the authors believe that many young people are overweight not because of troubling psychological and behavioral factors but because they simply were unable to locate recipies for pin a colada milkshakes, cornish hen, skewered shrimp and frozen banana chips. These are among the 30-odd recipes in one section of the book.

The book is divided into various admonishing chapters. The chapter titles are an accurate reflection of their contents. Overweight children are hardly likely to warm up to "Fat Isn't Fun" and "Oops."

Although the yuppie mystique is certainly an appealing one, it is inappropriate to apply it to the painful and complex area of weight problems in adolescents. "You Can Do It!" assumes that obesity is partially a result of wrong ordering in restaurants and using food as a crutch at parties or summer camp.

It does an enormous injustice to obese children to fling this sort of simplistic nonsense at them. Sample menus with broiled tomatos "italiano" and zucchini and carrots are about as therapeutic as telling a heroin addict to switch to aspirin.

It is troubling that a serious guidebook to weight reduction for children should be undertaken without a physician coauthor. Although we have been led to believe that there are perils in dieting without supervision (and emphatically so in the case of youngsters), the authors of this new entry are, respectively, a successful dieter, a free-lance journalist and a writer whose previous titles include "So You Think You Know Your Parents" and "Limer-Wrecks."

The vision of chubby 13-year-olds standing at the stove sprinkling oregano and freshly ground pepper on their lean loin lamb chops is a difficult one to conceive. But not for these authors. Their 14-day menu plan is rife with such gastronomic impositions on the hoped for legions of "The You Can Do It!" diet readers.

The notion of helping children with overweight tendencies is certainly sound. Giving them this book, however, is more likely to underscore their existing sense of isolation and loneliness.