BOSTON — WHAT makes Frank Shorter run - and run and run and run? Nearly two decades after his greatest triumph, a victory in the 1972 Olympic marathon, Mr. Shorter's enthusiasm for his sport is virtually unabated."Running is part of my routine; it's really the activity I enjoy the most," he said in a recent interview here. "People say to me, 'How do you motivate yourself to go out there every day?' In a way I run now ... the same way I did when I was in college [Yale]. At 3 o'clock, if I had done all my work, I could be rewarded by working out." On this occasion, Shorter's "reward" must wait until he completes an early-afternoon appointment as a spokesman for the the Alamo Alumni Run, a series of races in which runners compete individually and for their alma maters in cities nationwide. The Boulder, Colo., resident earned a law degree and passed the bar after the 1976 Montreal Olympics, where he was a silver medalist in the marathon. Still wishing to compete, but lacking professional opportunities to do so, he turned to opening up what he saw as the sport's antiquated rules. First, he used his position as a budding sportswear entrepreneur to fight for athlete endorsements. He refused to accept an international ban as a "professional," and "in essence said [to the international governing body], 'Look, what if it is my business? I capitalize it; I work at it actively; how can you deny me the right to put my name on it the way anyone would put their name on a business if it were theirs? He cites the financial support some other countries provide their athletes. Since then, the restrictions on endorsements have come tumbling down. Today, he says he's a businessman, TV commentator, and attorney - in that order. Running is an avocation, a lifestyle. Boulder-based Frank Shorter Sportswear has been a going concern worldwide. Shorter has always enjoyed the design work, and boasts that he's modified every US team uniform ever issued him: "In distance running there was some real validity to doing that in the '70s because a lot of the singlets were very hot and didn't breathe." Shorter's company doesn't sell running shoes, but he has some advice: "Nothing 'breaks in' running shoes," he warns. "It never feels any better than the first step you take in it." On technique: "Don't think about it. Everyone is put together a little differently." And on training: "Many people think to improve they have to be going as hard as they can, which is not true. Eighty to 90 percent of your running is done at a pace anybody could do because your conditioning pace is a very moderate pace."