Like the million other borderline flabbies who put Jane Fonda's Workout Book on the best-seller lists and have kept it there going on 80 weeks, Linda Sunshine started the hard way. A slightly zaftig brunette, she was stoically pumping and grinding her way through Fonda's videotaped exercise regimen one day when she felt the stirrings of breathless rebellion. "We got to the advanced exercises, and it was a killer," she recalls. "There's one called Rover's Revenge where you're on all fours and you have to lift your leg. I started to cry."

Soon afterward, on the theory that misery loves company, Linda, 35, began to wonder if her fellow Fonda sufferers might not buy a spoof on the subject. A former book editor, Sunshine (yes, it's her real name) decided to approach friends at Bantam. Nine weeks later she had 500,000 copies of Plain Jane Works Out ($3.95) in the stores.

Sunshine's slender 64-page best-selling paperback takes the relaxed approach toward exercise. A bed, she maintains, is a perfect place to work out, and if you should happen to doze off, don't worry. As for musical accompaniment, Linda suggests records that deaden the senses. "An album such as Richard Burton Recites the Spoon River Anthology is doubly effective," she points out.

Plain Jane's exercises are similarly unorthodox. To keep limber, Sunshine suggests the Refrigerator Lunge ("Lunge into refrigerator. Inspect every food item and select only the least nutritious and most fattening treat"). She also provides nutritional information (identifying the four basic food groups as White Bread, White Sugar, Soda Pop and Candy), sample menus (anything baked by Sara Lee is good for a midnight snack, she contends) and tips for exercise class (put simply: "Stay home"). Plain Jane is not, however, merely a put-on. "The philosophy behind it is that it's okay to be 10 pounds overweight," explains Linda. "Women should be able to say: 'This is what I am, this is what I look like, and that's fine.' "

The author herself is not so cavalier about her own barely noticeable avoirdupois. She carries 140 pounds on her 5'6" frame and feels she is 30 pounds overweight. "I really should have been a Rubens girl," she says. "I always thought I was born in the wrong century." She is a chronic dieter and has been ever since her teenage years in Fair Lawn, N.J. Her Jewish parents—mother Norma Sunshine is a sportswear designer, father Harold an optometrist—stood by as Linda periodically made attempts to trim off pounds by going on a water fast, taking diet pills, and practicing self-denial with Metrecal.

By 1970 Sunshine had dieted her way through Ithaca College and earned a degree in English. Later she became an editor at Crown Publishers, and in 1977 she was named editor-in-chief of Fireside Books, a division of Simon & Schuster. Sunshine quit in 1980 to try writing, turning out a children's book and a Silhouette Desire Romance entitled Constant Stranger.

With the fruits of her latest labor, Sunshine has no plans for buying a Nautilus for her rented apartment in Greenwich Village. She's more interested in her next venture into print, the Plain Jane Workout Calendar for 1984, due out in September. It will commemorate such gastronomical milestones as the consumption of the first Twinkie in 1930.