As the youngest female runner in the field, Emily Cox (right) has been training hard for the Boston Marathon, waking up at 3:45 a.m. during the week to run before school. Her grandfather, John Cox Jr. (left), was her inspiration to begin running marathons. (Nick Plum for Synthesis/Koubaroulis LLC./The Washington Post)

No one would fault Emily Cox if she decided to slack off a bit as second-semester seniors sometimes tend to do. She has taken and excelled in her courses at W.T. Woodson (including 12 Advanced Placement classes). She has found her passion in art history. The college acceptance letters are in — Virginia, William and Mary and Brown, with wait-list spots at Harvard and Columbia.

But the 18-year-old Fairfax native is still getting up at 3:45 a.m. to run 10 miles before school. On Monday, Cox will compete in the 118th Boston Marathon as the youngest female runner in a field of 36,000.

She runs long distances for the same reason as many runners — for her, it is a daily affirmation. It’s something she does for herself. It’s about the moment and about having some time, even if it’s just 90 minutes, when she is in control. It relieves stress from her academics and the pressures of daily life.

She runs races to be a part of something bigger than herself.

Her passion for running started three years ago when she joined a family tradition of running the Marine Corps Marathon.

W.T. Woodson senior Emily Cox trains for her first Boston Marathon by running before or after school in her Fairfax neighborhood. (John McDonnell/The Washington Post)

Cox set a goal to qualify for Boston because of her competitiveness and desire to be a part of one of the biggest marathons in the world.

She earned her spot after months of training — most of it before the rest of her classmates had woken up — and a focus that rivals athletes twice her age. Cox doesn’t have Facebook, Twitter, any type of social media or a smart phone. She doesn’t like to listen to music during races or training.

Running in the first Boston Marathon since the 2013 attack just means Cox will be more focused on her surroundings than usual, to take in the emotion of the race and remember why she and her fellow competitors are running .

“I’ll have something to run for this year, more than me just trying to finish a marathon,” she said. “Ultimately I know I’ve trained well. I have to trust my training and have fun. I want to have a smart pace, go out and just enjoy it, enjoy the experience, enjoy Boston and be part of something bigger than myself.”

Cox signed up for her first marathon to honor her grandfather John Jr., the man who got the family running. Nine relatives on her father’s side of the family have combined to complete 101 Marine Corps Marathons — and that doesn’t include her mother’s side. She’s the oldest of four girls, and her younger sister, Sarah, a freshman at Woodson, has already run two Marine Corps. Cox is the first from either side of the family to race in Boston.

John Jr. was a World War II Marine who fought at Iwo Jima and was the last WWII veteran to run the race. He ran his first race in 1986 with his son — Emily’s father — John III. John III hasn’t missed a race since. He once left his brother’s wedding in New Jersey at 1 a.m. to drive back in time for the 7:55 a.m. start, all to keep the family streak from ending.

John Jr.’s doctor finally convinced him to stop racing at age 84 after he had a pacemaker inserted, and he passed away two years later in August 2011.

Cox, who was 15 at the time, decided to compete in the following Marine Corps in her grandfather’s honor, even though she had never run more than three miles at a time. The age minimum for the Marine Corps is 14.

“I needed some sort of divine inspiration there,” she said. “And that was it.”

The family’s prolific attendance and John Jr.’s legacy allowed Cox to get a last-second spot in the 2011 edition just two months before race day.

She finished in 4 hours 35 minutes 8 seconds without training, a testament to the endurance she had built from years of competitive tennis. John III had looked around for her anxiously at the start line, only to turn around and find that she had taken a step back from the family.

“I asked her why she stepped back, and she said, ‘Well, I want to make sure I pass you!’ ” he recalled.

Cox played national-level tennis through middle school but left the sport in ninth grade after deciding to focus on academics instead of an athletic scholarship. Her grandfather was the first person she called after every match, win or lose.

She ran for Woodson for one season after her first Marine Corps — she had caught the eye of the coach after lapping the outdoor track team during a timed mile at tennis practice. But it took up too much time and created too much outside pressure. The day after her sophomore year ended, Cox started running nine miles a day.

She finished the 2012 race — her second marathon — in 3:31.51, placing second in her age group and qualifying her for Boston, which has an age minimum of 18. She qualified for the 2013 edition but did not turn 18 until March 30 of this year.

“You see the work that goes into making that happen, and from a parent’s point of view, I’m just so proud of all the effort and the dedication she’s put into participating in the event,” John III said, “especially this year.”

John III has sacrificed his own training to facilitate his daughter’s . It’s worth his slower finish times to get the hours alone with his eldest child.

Cox likes to get the long runs done and out of the way by 5:30 a.m. John III wears a head and bike lamp to cycle alongside her to their halfway point, at Zoe’s Kitchen five miles down the road in Fairfax.

Sometimes they talk, and sometimes they just enjoy what John III calls “the beauty of the morning runs, the quiet solitude.” They see the same delivery men, the same homeless people reading early editions of the newspaper, the same fire marshals waiting for Dunkin’ Donuts to open — “a whole other population of people,” John III said.

He hopes to kick his training back into gear to join Emily at Boston one year. John Jr. loved to run, but for him, “the marathon was really a vehicle to see the family,” his granddaughter said.

That she is the first of the family to qualify for Boston and one of the youngest runners would come as no surprise to him, given how his granddaughter's competitive streak rivaled his own.

“He would be really proud and really supportive,” Cox said. “This is totally something he probably would have expected.”