MONDAYS IS the start of finals for second-year law students at Catholic University, a nerve-shattering moment of truth for hundreds of young men and women.

Nan Shepard won't be there. She won a brief reprieve so she can face her own moment of truth, a challenge measured in miles and feet instead of facts and figures.

Shepard is 34. On Monday she will take her tiny 5-foot-1, 110-pound frame along the hilly back roads and downtown boulevards that consitutute an amateur running's most glamorous event in this country - the Boston Marathon.

It will be only the second full marathon for Shepard, who qualified for Boston in her first test, the Washington's Birthday Marathon at Beltsville. She ran that gruelling 26-miler in 3:21 and was the second woman to finish. But there is no Heartbreak Hill at Beltsville.

"I've always wanted to go to Boston," said Shepard, who's been running since 1966 but had never raced until last January. "It's the one thing in my whole life I've always wanted to do, but I just never knew I could. . . .

"The first time I raced was at American University, on a 25-kilometer course - that's about 15 miles - and I was doing okay when one of the men runners told me that if I passed a woman who was just a little ahead of me, I could take second place. And when I did it really helped me think I could do it."

With Boston as her goal, the uncoached Shepard sought outside help by joining the Washington Runners Unlimited, an organization devoted to encouraging women runners in the metro area.

In that group she found the closest she'd come to a coach in Fred Roughton, the only male member of the club and husband of president Henley Roughton.

Shepard told him she wanted to qualify for Boston in the upcoming Beltsville race by running under 3 1/2 hours, the cutoff to qualify.

"She had no idea what pace she could keep to qualify and was dubious about her chances," Roughton said. "Her only problem was that she had practically no experience and no benchmarks in other races to follow. So we talked and determined what pace she could do based on her time at AU and she did it exactly, right to the letter, the whole way."

A common mistake inexperienced runners make in distance running is to go out too fast and gradually slow down. That, according to Roughton, a self-taught coach who's provided guidance to several Runhers, is the "painful way to go."

"Besides, it gives a runner a tremendous mental lift in the final third of a race to be passing people instead of being passed. That's when a steady pace throughout really pays off."

Steady is how Nan Shepard trains and her consistency probably accounts for here success in her first few races, despite her lack of coaching. She has averaged 10 to 15 practice miles a day for two years.

Before racing became her motivation she had enjoyed running as an emotional release from the tensions of her job as a social worker for the District government. After returning to school she found her mileage routine helped her studying.

"Nan's done more than enough quantity training," Roughton said. "She has trememdous potential to improve by adding quality to the quantity and getting some racing experience and speed training."

One of her classmates at CU Law, Ben Beach, has run the Boston marathon 10 times. Asked about Shepard's chances, he assessed her training as "pretty damn diligent" and her qualifying time of 3:21 as "awfully good for a first effort."

"Nan's built light," Beach noted, "which is a good thing for marathoners because then they don't have to carry around any extra weight.

"Nan's just beginning to adjust her thinking a bit to realize her potential. Before Beltsville she didn't believe she could do as well as we were telling her she'd do.

"It's refreshing to find someone who's pleasantly surprised that she's accomplishing things in running," Beach said.

"Refreshing" might also be a good word to describe Shepard's appearance. At 34 she looks more like 14, with strawberry blonde hair and freckles. She'd fit as a model for a natural cereal commercial - glowing complexion, bright blue eyes and energy to bunr.

Her life doesn't rise and fall on running, but she loves it and is quick to suggest "you should try it" to a non-runner. She has induced several friends to take it up or increase their distance.

"Oh, she's definitely affected me," said Carl Feldbaum, a 33-year-old lawyer who was jogging a few miles a day until he starting following Shepard's lead. "She's got me running distances now and she managed to provoke me into running in the Cherry Blossom (10-mile) race."

Feldbaum reflected on his first (and last, he says) 10-mile run with Shepard.

"At seven miles I was dieing, so finally I quit," he said. "Nan ran up the last hill to her house and I tried to hitch a ride, but no one would pick me up because I looked terrible. She ran me into the ground. When I finally walked the rest of the way and got to her place she was sitting there drinking a beer."

Shepard puts in most of her mileage on the towpath along the C&O Canal. She's met fellow runners en route who have now begun to give her tips on improving her style.

She's become such a familiar sight along the canal that one other regular there asked her, "Aren't you famous?"

Feldbaum chuckles about that.

"Well, Nan's probably one of the few people in Washington who really does deserve to be famous."