Olympic dreamers come in three classes. One is the group of young and enormously gifted innocents, mostly collegians such as Renaldo Nehemiah, who are sheltered and secure enough to fret only about perfection in their event.

Nehemiah knows how sweet sport can be. Friday night, he lowered his world indoor record for the 60-yard high hurdles by five-hundredths of a second, to 7.02.

"What's it like to be Nehemiah?" said Marty Liquori, who broke four minutes in the mile as a 17-year-old 12 years ago. "At that age, you're totally wrapped up in track and field, you're able to keep everything outside away -- and he has a chance to be the greatest hurdler ever.

"But you don't think you're ever going to get injured, and you're really not a great athelete until you come back from a serious injury. Sooner or later that happens, and your life changes a little bit."

Liquori is part of the smallest class of runners, jumpers and throwers, a man who has used semiamateur sport to become financially comfortable, through a chain of runneroriented stores.

He also knows how cruel sport can be, having missed the last two Olympic teams because of injuries after making the '68 team as a teen-ager. And how unfair, having watched the once-peerless George Young lose his chance at gold the day the '68 Games were awarded to the rarified air of Mexico City.

Between the Nehemiah-like prodigies and the Liquori and Lasse Viren-like fossils of athletics, comes the largest of the classes, the postcollegians who must try ot meld fantasy with reality. Or, as Mark Belger might put it, how to prepare for Moscow while doing the laundry.

"What I'll do often is put a load of clothes in the washer and go out on a four-mile run," he said during the CYO meet Friday in Cole Field House. "Then I come back, put the clothes in the dryer and then do striders in the parking lot.

"It's quite a nice parking lot, L-shaped and maybe 330 yards of strightaway. Also, I run at night a lot. The other night in Washington, I ran along the other night in Washington, I ran along the Potomac somewhere. And some other really dark area I crept through."

Belger's visions are the 800 and 1,500 meters is the '80 Olympics; he has managed to combine training and earning a living about as well as anyone in his class.

"I'd been recruited since may sophomore year in high school," he said. "And I made up my mind to go to a college and get something out of it beyond running. So I went to Villanova, ran and also learned something.

"When you graduate, usually there are two alternatives. Either you quit training or you settle for a low-paying job. When I left Villanova (last May) with a B.S. in marketing, most of the people in my field were starting at $10,000 or $12,000 a year.

"I wanted to skip that, and still devote 40 hours or so a week to training."

Data Resources of Lexingon, Mass., has been the answer. But there has been a price: odd hours, which he regards as a blessing, and the sort of travel pace rarely seen outside pro basketball.

"I had two appointments before the meet, at 10 and 2," he said after winning the half-mile in 1:48.9. "Funny thing, I skipped the third one so I could get in a nap, and it was cut short because the mayor was coming out here for the meet."

Belger will be in New York on Monday and in Washington the next few days on busines, trying to slip in a workout whenever possible. Then he will compete in Philadelphia Friday and New York Saturday, fly home to Boston Sunday and leave for San Francisco the next day.

"It's something I've been expecting for years," he said. "I've adjusted quite readily. There is no way I would want a nine-to-five job. I'm flexible. I work Saturdays sometimes and also Sundays, 55 hours a week usually and still train 30 to 40 hours."

Liquori would raise an eyebrow ever so slightly about now.

"Belger will never be a miler," he told Track & Field News not long ago. "I don't think the has the interest, and he's just too heavy. You'd have to know Mark. He's not found of hard work; he's just fond of winning races. He just runs on guts."

Liquori, Belger and Nehemiah all were on hand at the CYO meet, three men who together form a running spectrum of experience and development. They are dissimilar in so many ways, yet have the same goal: Moscow.

"I'm at 1:45 now (for 800 meters)," Belger said, "and that's not good enough. I've got to get to 1:43, and it has to come through strength and stamina.

"I can to 1:44 through working on strength. Then I'll go for speed." If the dry cycle lasts long enough.