W.T. Woodson senior Emily Cox, shown training earlier this month, completed her first Boston Marathon on Monday. (John McDonnell / The Washington Post)

Emily Cox had heard that your first Boston Marathon is always the hardest. Physically, the course was just as challenging as advertised, the W.T. Woodson senior said. Emotionally, it was everything she expected out of her first Boston and the first race since the 2013 finish line bombing.

Cox, the youngest female competitor in a field of 36,000, thought Heartbreak Hill would be the toughest part of the race. Instead, a man at the top hosing down passing runners made it one of the more relieving parts of the course. The next six or so miles were the most painful.

“My legs were not happy by that point,” Cox said. “But you felt like you couldn’t start walking, just because of all the spectators.”

Cox finished in 3:46:08. She qualified for next year’s Boston Marathon with her finish time in the 2013 Marine Corps Marathon, which allowed her to focus on the experience of this year’s race instead of her time. The Marine Corps, which Cox has run three times, draws the most spectators near Georgetown. Cox said every mile of the Boston course was as packed as that span of the Marine Corps. Reports estimated that more than one million people came out to line the Boston route this year, twice the usual number of spectators.

“There were so many people thanking the runners, yelling ‘Boston Strong,’– it was an incredible sense of community, a once in a lifetime opportunity,” she said. “Next time I run I want to put my name of my shirt, cause I’m pretty sure the people that did had their name cheered by 4,000 different people.”

She realized at one point she was running near charity runners on a team for Martin Richard, the eight-year-old killed in last year’s attack. It was a reminder for her of the people running for bigger causes this year and motivation for her to keep going as her legs became more and more tired. Everyone picked up the pace despite the pain and sped toward Boylston Street as they neared the finish line.

“Knowing that you’ve done all the training, all the community has been through — it’s just the entire experience packed in a nutshell right at the end,” Cox said. “I was an emotional mess after.”

She got her first look inside Fenway Park during the post-race party and is heading straight from Boston to Providence, where she will attend Brown’s admitted students day — after she hikes up the hill from the train station to campus on sore legs.

More: Emily Cox runs for ‘something bigger’

Video: Cox explains her early-morning running routine