He reached a personal best with his time at the Boston Marathon, but his real motivation is his son, Jack, diagnosed with autism.
"He's 7 and still in diapers. He's not really talking. He's got speech but he doesn't really talk. We've never really conversed with him," said Felsenfeld.
He set out to raise money and awareness for children with autism, fundraising for the non-profit organization, Train 4 Autism. Felsenfeld has finished at least one marathon a week for the past year, logging nearly 3000 miles.
Juggling work as a web developer, Felsenfeld's year-long challenge helped raise more than $85,000 for autism-related charities.
For the past year, his wife Tiffany has cheered him on at each finish line.
"When he sets out to do something, he'll do it," said Tiffany.
His last race was the Operation Jack Marathon last weekend in Manhattan Beach.
"It was really exciting to see Jack with a huge smile on his face," said Tiffany.
With the challenge over, Felsenfeld says the legacy will live on for the little boy who inspired his father.
"He's got a lot of fans and he has no idea. I wouldn't be doing it if not for him," he said.