Denise Austin, apostle of rock aerobics, is thinking about her tummy.

"You gotta feel my stomach!" she shrieks at the door of her Alexandria town house. "I think you'll be amazed."

Whisking off a bright blue Reebok sweat shirt, the "Today" show's fitness expert reveals muscles of granite rippling beneath her yellow leotard. Hoping to become the Jack LaLanne of her generation, Austin has transformed her 5-foot-4 body into a conglomerate that would make Lee Iacocca proud.

"I've worked out with the world," says the 30-year-old "fitness personality," who lists among her accomplishments that she was the first woman to do aerobics live on American television from Moscow. Today, as part of Nancy Reagan's "Just Say No" antidrug program for children, she'll lead about 35,000 spectators at Yankee Stadium in a minute of pregame exercises.

Although best known for her spots on NBC's "Today," Austin has traveled the country ("every state except Alaska") and the world ("Russian women just don't care how they look") preaching the gospel of physiological fortitude. "The Jane [Fonda] videos have better sales, but I'm in this for life." Politics don't interest her much ("It's too serious, I'm really more into body-fat testing") but she has tutored Ethel Kennedy, whom she refers to as "determined."

Her "Rock Aerobics" and "Hot Legs" workout tapes are best sellers, and she earns as much as $3,000 a day teaching corporate America the ins and outs of her high-tech exercise guide, "Tone Up at the Terminals." Whenever she is on a plane she takes along "Flying Fit," the pamphlet she wrote for Trans World Airlines.

People want what Austin has to offer. After she unveiled tips from her latest video, "Rock Hard Tummies," on "Today" three weeks ago, the show received more than 20,000 letters from flaccid fans seeking help.

"No doubt about it," said Steve Friedman, the show's executive producer. "She comes across on the tube. Now, I find the fact that somebody likes bending over and picking up weights a little bizarre, but that's Denise."

Denise and her husband Jeff Austin (brother of tennis star Tracy) moved to the Washington area three years ago from Los Angeles, where she was raised. He's an attorney and works for Advantage International, a D.C. firm that represents sports stars Dan Marino, Moses Malone and Hana Mandlikova, among others.

The Austins rank Jimmy Connors among their closest friends ("I just don't think he trusts anyone like he trusts Jeff," Denise says), and their active social life often includes dinners with Vice President George Bush's son Marvin and his wife Margaret, and Urban Mass Transportation Administration chief Ralph Stanley.

Washington has been a big switch for Denise Austin, but one she says she welcomes. "I really can tell the difference in East Coast people," she says. "Here people are more into their intellect -- they are thinking important things. In L.A. it's body, body, body.

"Back in L.A. none of my friends were too stressed-out. Jeff works till 7:30 every night now. In L.A. he would be off by 5."

Austin says she wants to have children someday, but realizes it will call for some unusual sacrifices. "I don't know if I could have a rock-hard tummy after that."

Always on the move, she swims, bikes, plays tennis and jumps rope -- and "those are just for fun." For her "Today" appearances she flies to New York the night before and is up at 5 a.m. She arranges most of her public appearances herself, because, she says, "I just feel like I can get the job done, I have that energy going."

Although she works out vigorously and constantly, she has no time for people who are too single-minded about exercise. "I don't want to be taken too seriously," she says. "Fitness is a part of life, like brushing your teeth. I like to party. I even have a bag of Fritos right in my own kitchen."

So why does she do it? Is it health that compels her to perform as many sit-ups each morning as other people do in a lifetime?

"Mostly, I like to feel good and to look good," she says. "I never think of heart disease when I'm jumping rope.

"Everybody today knows when you feel better you can do your best, and doing your best is the American way."

Austin considers herself a motivator, and she's committed to helping the staff of "Today" feel better. "Jane Pauley does her exercises," she squeals. "She has great legs. Bryant [Gumble] is a little cynical about it. He's not really an exerciser, but he's trying. I just got him a bike.

"Willard [Scott] is the hardest. I sent him a pair of Reeboks and he promised me he'd walk every day. Same with Gene Shalit."

Austin is in such demand today that she needs two assistants to help her with mail and telephone calls, but that wasn't always the case. She attended the University of Arizona on a gymnastics scholarship, and when she graduated in 1979 with a degree in exercise physiology she headed for Los Angeles.

"It was tough at first," she says of her attempts to market herself as an aerobics expert. "People treated me like a nut or something." She finally got a few spots on local television, and then her own exercise show, "Daybreak, L.A." Her big break came when she appeared on "The Merv Griffin Show."

"Merv put me on the map," Austin says.

She hopes to do more television work in the future, possibly sports reporting. But for now she says she's happy with the 42 projects she is involved in. She just bought a silver Saab with heated seats ("I guess I'm the typical yuppie"), and she also donates time to charity and antidrug work with youngsters.

"Life is so great here," she says as a Federal Express delivery man drops off a box of her brand-new tummy tapes. "I just want people to know that. I really think there are enough dull and bored people out there that are upset or worried. I want them to know life is a little better than they think it is. I just want to go out there and cheer them up."