A year later, when she thinks of Krystle, it brings her peace.
"I talk about how it changed my life, being a little bit more grateful and slowing down a little bit more and trying to see the more positive things in life than the negative," she said.
"I also feel so much calmer. I feel like when you're caught up in the regular pace of life and you work and you have kids and your life is so busy and it's so hectic and you feel like sometimes your blood pressure is way up here," she said, raising a hand up, up, up. "Everything for me is so much calmer."
In the 12 months since she lost her leg and her best friend, Karen has healed, rehabilitated and learned to walk again. Through a chance encounter, she's used her experience as an amputee to help a 15-year-old girl in El Salvador get to the United States, where she's received treatment and been fitted for a prosthesis of her own.
Karen also has a new last name. In March, she married Kevin McWatters, the man she was at the finish line to see.
Kevin will be headed down Boylston again soon, running the marathon one last time, to finish what he started last year, and to put it all behind him.
"I don't want to let them terrorists win," McWatters said. "Like [David] Ortiz says, not in our bleepin' city. No way, I'm going back. Just one more time, just so I can just complete it because last year was gonna be my last year.
"I just want to go back and finish it just for my mental health."
Karen won't be there; she has no interest in going back and reliving it all. She's moving on, not looking back.
"I feel like I'm just over getting caught up in it," she said.
But all of the other people ESPNBoston.com talked to will return to Boylston Street on April 21. Mercurio and Wright are running again. Evans and Hooley will man their stations. Landry will be back, volunteering in the medical tent. Menino will be back, though he's not sure where. And Carlos and Melida Arredondo will be by the finish line, cheering on the runners.
Every one of them remembers vividly where they were that day, a year ago, at 2:49 p.m. They understand they'll likely never forget it.
None of them knows exactly what to expect on April 21. They know the day will be fraught with emotion, thick with memories that will mean something different to each person.
But in the end, the only way to know for sure what it will be like is to be there.
"It's going to be a day to gather together and cheer and remember," Arredondo said, "and try to figure out where we are and where we're going at that point."