Long-distance runners like to think of themselves as athletes who perform in splendid isolation. But the people who watch, like the scattered few thousand who turned out for the Marine Corps Marathon yesterday, know better.

"After 21 miles, it's all emotions and guts," said Michael Asip, the fan behind marathoner Don Greco. "That's what pulls you through the finish line and people can really make the difference."

Supporting a marathoner is a skill that, like running, demands good timing, patience and physical endurance. Many relatives, friends and fans who greeted the runners at the finish line near the Iwo Jima Memorial yesterday had learned how to wait, bolster, congratulate and comfort.

Some, too, knew when to remain silent.

"My company is what's needed most, not my gushy words," said Barbara Prowse, as she waited for her husband, Bert, to snap out of his exhausted stupor. "My husband knows I'm proud of him."

Her husband, flat on his back and eyes closed, found just enough energy to acknowledge the tacit understanding he shared with his wife: "You just don't say something for a while."

As they came over the finish line, many of the runners, winking sweat from their eyes, quickly lifted their heads to look over the crowd for a glimpse of their fans. "She's my trainer, my coach, my doctor," said an exultant Don Ciccarelli as he looked over to his wife, Debbie.

Asip, Pandora Lamb and Marsha Goldberg were among those who seemed to have expended as much energy cheering their friend Greco as he did running the race. Wearing running shoes themselves, they had followed him for much of the 26.2-mile course, waving signs like "Go, Don, Go" along the way.

The first time Greco saw his friends, he slapped their hands in appreciation, Asip said. But the last time, five miles before the finish line, he had something more urgent on his mind. "He asked me to run with him," said Asip. So Asip joined him for the last three miles.

That turned out to be more than physically demanding. "He asked me to talk to him," said Asip. "He said, 'Mike, please talk to me,' because he wanted to get his mind off the body and the pain. So I talked to him about politics."

Asip saw his chance. "Greco's been supporting (President) Reagan," he said, "so I tried to talk him into voting for (Democratic challenger Walter F.) Mondale while his mind was weak." Asip said he did not succeed.

Karen Lovelace said she spotted her son-in-law in the crowd and implored him to pull her ahead. "He came and held my hand, and I said, 'I don't want you to hold my hand, I mean pull me.' So he pulled me along, for a half-mile at least."