Give people a hobby, and someone will spend a fortune pursuing it. And someone else will turn it into a thriving business. Tom Gilligan, a running enthusiast in Cambridge, Mass., did the latter.

When Gilligan entered the travel business nine years ago, jogging was an activity he did in his spare time. Then he began joining competitions and meeting other participants. In 1977, a group of runners asked him to arrange a trip to Hawaii so they could run in the Honolulu Marathon. Gilligan knew an opportunity when it came his way. He became a specialist in tours to far-off footraces.

For two years he built up his clientele while working at an agency. Then in 1979 he took his loyal customers with him and opened Marathon Tours Inc. Fir his initial working capital he used his Master Charge card to pay a $1,000 deposit on a group hotel reservation for the New York Marathon. Now he is grossing about $400,000 annually and has recently led groups to races in Shanghai, London, Montreal, and Stockholm. In April Gilligan handled arrangements for about 250 runners entering the Boston Marathon from other cities. In September, he will lead a tour to the Cayman Islands in the Caribbean.

With only two full-time employees and one part-timer, plus a consultant for his computer operation and a nurse who accompanies each group, the business is still tiny. "I'm not paying myself a full salary yet," Gilligan says. "Everything I make is going back into the business -- into a larger office, higher salaries, and publicity. I don't want to show a profit until the year after next."

So far, Gilligan says, he doesn't face effective competition. Other tour operators tried making the same kinds of arrangements, but since they weren't runners themselves, they lacked firsthand knowledge of the interests of runners. Gilligan can advise customers on the quality of a particular course and on the care with which an event has been organized. He avoids races that are inaccurately measured or that don't have sufficient water or medical stations. The best ones, he says, have a variety of ancillary activities: carbohydrateloading dinners the night before, videotaping, awards ceremonies, and parties for meeting local participants.