Hal Holbrook, whose award-winning one-man show, "Mark Twain Tonight!", returns to the Kennedy Center Opera House this week, is also a passionate solo performer as a sailor. The actor has been sailing since 1968, when he rented a house in Greenwich, Conn., while performing in New York in "Man of La Mancha." The house had a dock and a powerboat, and Holbrook, who had sailed only once some 10 years earlier on a charter boat in the Virgin Islands, bought a dingy for recreation. "It got into my blood," says Holbrook, who has now done considerable coastal sailing on the Atlantic, and has put in some 20,000 miles on the Pacific.

From the start of his love affair with the water, he has frequently been a solo sailor. "I'm not in a business with 9-to-5 jobs, and I don't like to wait for people to be available. I learned from the beginning -- under all conditions and making a lot of mistakes -- to be alone.

"It's very beautiful out there alone. It can be slightly frightening, but you learn to deal with all the fears you have inside yourself. It's a matter of trying to do something that very few people would dare do. It's tremendously exciting -- a great challenge to your health, your spirit, and your belief in yourself." Among his favorite sailing trips are:

* The first trips he undertook, mostly in Long Island Sound, as he managed to figure out how to sail, and graduated from a dinghy to a 22-foot boat to 30-foot French boat. (He currently owns a 40-foot boat, The Yankee Tar).

* A solo race in June, 1980 from San Francisco to Hanalei Bay off the Hawaiian island of Kauai: "A gale hit us the first night out, and several boats dropped out. I had never been in a race before; it's hard because you have to keep the boat moving. It took me 22 days and was a tremendous challenge -- there was no contact with the land whatsoever; nobody knew where I was. But I wasn't frightened by the time I left. Before you take a trip like that you've already had your nightmares."

* A 300-mile non-stop qualifying course for the San Francisco to Kauai race: "It was the first time I had the experience of staying up and sailing day and night. I hallucinated. I was totally convinced I'd seen flying saucers, but I was so busy sailing, I had no time to deal with the phenomena of what at the time all seemed very rational."

* An extended sailing trip with friends from Hawaii to Tahiti to Pago Pago: "It was another world, totally beautiful and unspoiled. It was only a 400- or 500-mile trip, but it was gorgeous water; we cruised among the native villages, and had a wonderful time."

* A trip from the Northern Tongas to New Zealand with friends on The Yankee Tar: "We did fast passage -- 1,500 miles to Auckland in 15 days. It was an exciting and wonderful trip."