"Acting is a marathon," said Bruce Dern, who has done some running and some acting in his time. "The race doesn't start until everybody's gone 16 miles. Everybody can go the first 16 miles. The question is, what do you do in the last 10?

"Every actor can talk about losing 20 or 30 parts. It's what you do with the parts you get that counts. Everybody gets at least one at-bat," he said, mixing the sports metaphors. "If you're good you get two. If you're really good you get more, and if you get hits you make the team. I like to think I've made the major leagues and a few all-star games.

" 'Space' is a big at-bat for me. It's an audience that will be 10 times larger than the audiences for all my movies."

"Space" is a big thing for everyone. How big is it? Big enough to require two directors (Joseph Sargent and Lee Philips). Big enough to have been filmed in seven locations in the United States and one in England. Big in subject matter (the American space-exploration program) and time frame (from the end of World War II into the early '70s). It's taken from a big book, in both sales and size, James Michener's novelization of the space program. And it's big enough to take up 13 hours of prime time tonight through Thursday on CBS.

The cast is an imposing one, led by James Garner playing a naval officer who emerges from World War II as a hero, goes on to win a Senate seat and ties in politically to the space program. Martin Balsam is his political mentor. Susan Anspach plays Garner's emotionally ragged wife; Beau Bridges and Harry Hamlin play astronauts; Blair Brown plays Hamlin's wife; Michael York is a scientist who works on rocket development, first for Hitler and then for America; Barbara Sukowa plays York's wife, a woman who defies the Nazis and then tries to adapt to life in these United States; and Melinda Dillon plays Dern's wife, a woman who had to adjust to the disarray of the post-war world and her husband's deepening involvement in the space program.

And, as he said, it is a big role in a big production for Dern, an actor who has a history of playing big parts in little movies and little parts in big ones.

He plays Stanley Mott, an engineer who gains entry to the space effort when he takes on the task of delivering Hitler's rocket experts into American hands.

"The thing that excited me about 'Space' were its non- obvious love stories," said Dern. "In American space history the astronauts are the stars. But who put them in space? Who were those people and why did they do it? And at what cost?

"These people moved often, criss-crossed the country -- from Cape Canaveral to Houston to Huntsville to Edwards Air Force Base to Patuxent -- and it was only going to lead to some sort of family upheaval. Those were the things that interested me about 'Space.' "

One of the understated love stories revolves around Dern's character and his son, who embarrasses Dad by going to Canada to avoid serving in Viet Nam.

Ah, yes. The rebelliousness of youth. Like running and acting, Dern knows something about that, too.

It's not the easiest thing to become an actor if you were born the way Bruce Dern was, which is to say, well.

Bruce MacLeish Dern was born in Chicago 48 years ago. His father, John, was a lawyer whose partner in the office next door was Adlai Stevenson. His paternal grandfather, George Dern, was once governor of Utah and served as secretary of war under Franklin Roosevelt. His great uncle on his mother's side was poet Archibald MacLeish.

Dern attended Choate and New Trier High School and the University of Pennsylvania.

But when it came to acting, said Dern, such a background was no advantage. "When you have that kind of background," he said, "there are pressures from the family and community to go into one of their walks of life -- definitely not show business . . . Thirty years ago there was a big question about the legitimacy of show business."

The urge to act, Dern observed, usually comes with the rebellious stage of youth. For Dern, the revolution started his second year at Penn. "I was having a hard time communicating at home," he recalled. And he was having a hard time at Penn trying to major in journalism.

He had gone to Penn because his father and brother had gone to Penn, and besides he loved to run and the school had a terrific track coach. But journalism was another matter. Given a story about a boy whose dog had been killed, Dern recalled, "I poured my heart into the headline." "No, Mr. Dern," came the reaction from the grizzled night editor who taught the course. "The headline can only be one thing -- 'Dog Dies, Boy Cries.' " Maybe, the teacher suggested, Mr. Dern should try a line of wrk he could pour his heart into. Like what? Maybe novels? No, cautioned the professor, you can't spell.

With that, Dern left Penn and ended up in the Actors Studio.

His father died soon after Dern took up acting. "My mother always wondered why I wasn't getting Jimmy Stewart's roles my first year," he said. Much later he played a character he could have taken home to dinner. "I played Tom Buchanan in 'The Great Gatsby,' " he recalled. "That was the only acceptable role to the family."

There are, of course, advantages that come with Dern's pedigree. "It gives you a broad background in a lot of things," he said. "I'm close to being an expert in only one thing -- running -- but I know a little bit about a lot of things."

So there's a lot to draw on and bring to the screen, and if they ever do "The Jim Fixx Story," here's the man to play the part.

Dern started running when he was 9 years old, long before it became a fad. "I used to run marathons and ultra- marathons," he said. "When I lived in New York, I'd run around Manhattan. I'd run to Philadelphia. That's 91 miles." Now he's cut back and only runs 50 to 60 miles a week. "I've kept a log since I was 10," he said. "I just completed 106,000 miles."

Dern has also gained a reputation for being an expert on how to play crazy people. It started in the '60s when he played a psychotic hillbilly in an episode of "Alfred Hitchcock Presents."

"In those days," he recalled, "you got a job if you could walk in with a piece of film that showed you had a range. That episode showed I had range. Unfortunately I was often cast in those roles -- no one else would do them, or no one else could do them as well.

"I didn't think about career planning . . . I just thought that parts would get bigger and better . . . It took until 1970 or '72 for me to realize it was a business and you can't keep going on this way."

That was about the time he broke the mold by playing the impassioned -- but not crazy -- botonist who fights to preserve the last of the earth's vegetation specimens in "Silent Running." The film, which won a small but appreciative audience, was directed by Douglas Trumbull, who did the special effects for "2001" and "Close Encounters of the Third Kind." Two of the film's writers were Michael Cimino and Steve Bochco.

" 'Silent Running' began the movie-star phase of my career," said Dern. "Until then, I was just the ninth guy from the left. The thing that appealed to me about the film was the relationship between my character and the three drones, who were supposed to be mechanical but weren't."

The little drones and the botonist became like a man and three dogs marooned together. He gave them names -- Hughie, Dewey and Louie -- and talked to them. That relationship gave "Silent Running" a very human touch and kept it from becoming a typical sci-fi hardware show. "That's the same thing that interested me about 'Space,' " hesaid.

So, since "Silent Running," Dern's a reformed actor. "Only three times in the last 15 years have I played a guy who's over the edge," he said, sounding like a man who's kicked a bad habit, almost. "In 'The Cowboys,' my character was over the edge. 'Black Sunday' -- he was over the edge. 'Tattoo' -- he was way over the edge."

How about "Coming Home," in which he won an Oscar nomination for his portrayal of a Viet Nam veteran who walks into the water and drowns himself? "Well, there were reasons why he was the way he was," explained Dern. "The movie was about two kinds of people who came back from he war -- one with physical injuries and one with emotional problems. In the end . . . he takes a hike."

Dern points to five of his movies as his favorites -- "Smile," "Silent Running," "King of Marvin Gardens," "Middle Age Crazy," which won him the Canadian Oscar, and "That Championship Season," which earned him a Berlin Film Festival Award.

Any regrets over those years spent playing the deranged? "No, I never really look back," said Dern. "There's no other career or role I wish I'd had."