EVER WONDER how many countries a traveler would have to visit until there was no place new to go? The Travelers' Century Club, a national organization whose members must have stopped -- if only briefly -- in at least 100 countries, currently lists 309 international destinations that it regards as qualifying countries, island groups or other localities. (The United Nations, by the way, has 157 member countries.)

To set foot in all 309 is to triumph in the game of one-upmanship. Anytime the conversation turns to travel, you will always have been some place others have never seen. "Certainly the most exclusive travel group," says a club brochure, since time and money are essential to attain a perfect score. As far as anybody knows, that hasn't yet been achieved.

Some locations on the list are not actually countries in their own right, the Los Angeles-based club points out, but "they have been included because they are removed from parent countries, either geographically, politically or ethnologically."

An example is Easter Island, which belongs to Chile but is more than a four-hour jet flight off the Chilean coast. An applicant for membership who has visited both gets credited with two destinations. The same goes for someone who has visited both Asian and European Turkey. A swing around Antarctica would net 11 credits for each of the nations claiming territory there. Alaska and Hawaii count separately from the continental 48.

Founded in 1954, the 800-member organization annually sponsors a group tour to a location off the standard tourist path -- this year to Micronesia -- and holds dinner meetings in various cities in the United States. Initiation fee is $25 and annual dues, $15. To qualify, a visit can be as short as an airplane refueling stop.

The club's purpose, though, is not to collect countries, explains its president, historian Giles T. Brown, who is associate vice president of California State University at Fullerton (120 sites to his credit), but to promote international understanding through travel.

Nevertheless, some members do travel to pick up new destinations. One deceased vagabond, so club legend goes, made provisions in his will to have his ashes scattered in the places he had not reached while still alive. At the moment, says Brown, the most any still-breathing member has visited is 258.

Another organization that attracts the well-traveled is the Circumnavigators Club, an 850-member group based in New York. The primary qualification is to have circled the globe.

When the club was founded in 1902, that might have earned you a one-upmanship prize, too, but in the age of jet travel, says administrative secretary Helen Jost, an around-the-world trip has become fairly common.

Membership is made up chiefly of people in science, exploration and international affairs, says Jost. The club holds frequent dinner meetings for members and sponsors a travel scholarship for students in international relations. Initiation is $25, with a $75 annual fee, which tends to promote a certain exclusivity. New members must be proposed by a current member, but the club can arrange introductions.

For more information on the Travelers' Century Club: 8033 Sunset Blvd., Suite 9, Los Angeles, Calif. 90046. For the Circumnavigators Club: 24 E. 39th St., New York, N.Y. 10016 .

For the beginning or the serious runner, a 3-year-old Boston travel agency, Marathon Tours Inc., specializes in conducting trips to major races in this country and abroad. It's upcoming international calendar includes runs in Athens (Oct. 15), Dublin (Oct. 25-"pub-crawling" in Killarney and Limerick after the race), Bermuda (Jan. 29-30), Manila (Feb. 27), London (May 8) and Stockholm (June 4).

Last year, Marathon took 347 to the Bermuda 10-kilometer and 26-mile races and expects that number to grow to 400 for the four-day trip in January, says the firm's president, Tom Gilligan. (The cost of $432 per person includes Elbow Beach Hotel room, two meals daily, airfare.) A runner himself, Gilligan, 33, has competed in 24 marathons, with a best time of 2 hours and 32 minutes posted two years ago. He is currently vice president of the Greater Boston Track Club.

Tour packages generally include a vacation of a week or two plus a day of competition and may also offer training runs with local enthusiasts and, says Gilligan, "pre-race carbo-loading dinners." The charge for a week's trip to the Honolulu Marathon Dec. 12 is expected to be about $900, including airfare from the East Coast, airport transfers and hotel accommodations but no meals.

For information: Marathon Tours, 1430 Massachusetts Ave., Harvard Square, Cambridge, Mass. 02138 or (800) 343-5088 .

Hollywood fans can now visit the still-active studio lot of 20th Century-Fox for a 90-minute tour that includes a stop at the commissary, where lunch is available. There's always the chance of spotting a TV or movie celebrity who might be filming that day. In recent weeks, Richard Dreyfuss and Michael Douglas have been at work on the 60-acre site.

Unlike a visit to the Universal Pictures lot in Studio City, uhich is something of a movieland theme park, the 20th Century-Fox presentation is billed as "a walking tour of a working lot." A guide takes visitors down such 20th Century landmarks as "Hello, Dolly!" Street and Western Street as well as to current outdoor sets, including one where episodes of TV's M*A*S*H have been made.

The tours are available twice a day, morning and afternoon, and must be booked through Gray Line Tours, which will bus visitors to the studio in West Los Angeles from a number of pick-up points. The cost is $15.50 per person and is part of the line's Beverly Hills-West Los Angeles sightseeing package.

For information: Gray Line Tours, 1207 West Third Street, Los Angeles, Calif. 90017 or (800) 421-8921.