Originally from Russia, McFadden was born with spina bifida and abandoned in an orphanage, but after being adopted by an American family when she was six years old, she found purpose in sport.
"There were no doubts (about coming to London), McFadden, with a black ribbon pinned to her top, told a small group of reporters after a news conference in London.
"It's important as an elite runner, an elite runner with a disability, to be a role model for those who are newly injured and it's important for me to be an advocate for those.
"I go to hospitals, I go to schools, I go everywhere just to talk about disability and rebirthing life. I've lived with many challenges in my life, every single day and so I know something, of what it's like," added the Paralympian who won three golds on the track at last year's London Games.
"It's about nurture so that's what we need to do for the people of Boston, it's about nurturing and it's about rebirthing their lives after a tragedy."
McFadden crossed the Boston finish line several hours before the two explosions, which killed three people and injured more than 170, and watched on television from her hotel as the horrific events unfolded.
"We were watching the replay over and over and that was just the toughest part, to see the mad chaos, people running, people injured," McFadden said.
"That was so tough to watch, especially because the marathon is a community, it's separate from track and you get to know each other - why they're running, what charity they're running for and then you're explaining your story and it's a great experience."
The three-times Chicago Marathon winner was making her debut in Boston and her day had started with the nerves associated with racing.
"Monday was such a normal day you know, getting up at 4.30am, getting to the bus at 6.30 and being on that line at 9.17 thinking about how I'm going to run this race, having butterflies in my stomach," McFadden said.
However, there was no celebration after her victory and when firemen came to the hotel McFadden was staying in to make sure it was safe, the athlete's main concern became getting home to Baltimore.
"We packed up the fastest we've ever packed up and we were out of the city, she said.
"But what we can gain from this tragedy is the community of Boston came together.
"We're getting support from all around the world which is very important because it goes to show that we're not going to let this tragedy win.
"There's always going to be a few bad people in this world but the majority are good and it's about not letting those bad people win."
McFadden turns 24 on Sunday, the day of the London Marathon, but her mind will be on those in Boston.
"I think this Sunday, as I told my team mates and my parents and family, we'll be racing for the people in Boston," she told a news conference.
"I'll be carrying them in my heart as I am running through the course in London."
Boston men's champion Hiroyuki Yamamoto told Reuters he felt a special affinity with those whose loved ones had died because his 16-year-old son Kanta was killed in 2011.
"I was very sad to hear about the tragedy because innocent people got killed suddenly and it actually happened to my son two years ago - a drink driver hit my son," the Japanese said through a translator, adding that the memory of his son spurred him on during races.
"If my son wants to do something, he can't but I can, so whenever I get in a very tough situation, I can do it... It is a strong motivation for me."
There will be a 30-second silence before the race's mass start to mark the events in Boston and London Marathon organisers will donate £2 for every finisher to The One Fund Boston, set up to raise money for victims of the explosions.