Birch said that Evan left an imprint on her heart and his friends and family.
Donating his organs and participating in the run allows Casey to continue that legacy, she said.
"This little boy, he came into our lives and he just changed everything," Birch said. "When you know someone like Evan and you get to know someone like Evan, it changes a part of you and you look at people in the world differently.
"Knowing that she [Casey] could give some of him to someone else to help them, it just meant everything to her," she said.
Casey said she is no longer sad about the loss of her son, but glad about the lasting effect he has had on not only her and her family, but his donor recipients.
"We are not sad," Casey said. "Life on earth was not good for him."
Her son was unable to speak, walk or eat normally. He spent much of his life in a hospital as doctors struggled against the multiple medical conditions that cut his life short.
"I'm Catholic, so I very much rely on my faith," she said. "The one thing that got me through this at the end was the fact that, when Evan went to heaven, the very first thing he did was use his legs [for the first time] to walk to Jesus.
"For the little time that we had him, we are far better people for it," Casey said.