Preston Mack/Disney Sports
01/14/2013 at 06:45 PM EST
For two years, Joey Fatone cheered on his wife
, Kelly, as she ran the Goofy Challenge at Walt Disney World: a half marathon followed by a full marathon the following day. But when Kelly fractured her foot two months ago, Fatone decided to run in her place.
"I just said, 'I'll do it for you,'" he tells PEOPLE. "And it became something I really wanted to do for her."
on Dancing with the Stars
, Fatone, 35, had decided to get into shape.
"I wasn't training to run a marathon," he says. "I just wanted to start working out. I wanted to build more muscle and upper body strength. I have no definition in my chest! So that's how I started. Then I decided, 'Let's see how far I can run.' And it went from there."
The night before the marathon, Fatone did something that could have derailed the entire thing: at the birthday party for his 3-year-old daughter
, Kloey, he jumped on a trampoline for three hours. "I knew I could hurt myself," he says. "But it was her birthday. What was I going to do? I woke up the next day feeling really good and ready to run."
On Saturday, Kelly Fatone packed Joey a lunch – along with Advil and blister ointment. During the 13.1 mile trek, Fatone would run for a minute, followed by a minute of brisk walking. He finished the course in three hours, 10 minutes. "It really wasn't that bad," he says.
The next day, he tackled the 26.2-mile marathon. "I never thought, 'I'm not
going to do this,' but I did think, 'What am I doing?'" he says.
Around mile 18, his left knee started to throb. "I knew if I stopped, I'd be screwed," he says. "So I just kept going."
Finishing the marathon in six hours, 19 minutes, Fatone was triumphant. "Your body can do so much if you mentally prepare," he says. "I really do feel like I can do anything now. It's a great feeling."
On another hot topic, pal Justin Timberlake
's new song
"Suit & Tie," Fatone has nothing but praise for his former boy bandmate. "I like it," he says. "It's a different sound for him, but Justin's music grows on you. After you hear it a few times, you love it!"