Duncan, Oklahoma (CNN) -- Justin Boyles lay in bed, exhausted and depressed. His 385-pound body sank into the mattress with each breath he took. I have diabetes, he thought in disbelief.
His 6-year-old daughter, Autumn, crawled on top of the king-sized comforter and stared at him with bright blue eyes.
"Daddy, will you get up and play?"
"No, baby, I'm tired."
She sat back to think. Her mom had tried to explain why Justin was so sad, but Autumn didn't really understand diabetes or its dangerous implications.
"Daddy, are you ever going to be healthy enough to ride a bike with me?"
The question broke Justin's heart. Tears started to roll down his face. "Yes, very soon. I promise," he said.
"Do you pinky promise?" she asked, holding out her tiny finger.
Justin looped his last digit around hers and made the decision to change his life.
All kinds of excuses
Shanon and Justin Boyles were high school sweethearts. The first time they hung out at 16, Justin took Shanon to church. He was smart, polite and made her laugh -- three qualities that helped Shanon overlook his size.
"As long as I can remember, I've been overweight," Justin says. "I remember as a child coming home after school and having a stack of cookies and a giant glass of milk every day."
In high school Justin weighed close to 250 pounds; he was considered obese even at 6 feet 1. As an adult he lived on fast food and spent most of his nights as a security guard sitting down. He'd eat dinner at work -- frozen burritos, burgers and fries -- then stop off at Taco Bell on the way home for the advertised fourth meal.
"I was really kind of in my own little hole, and I didn't want to get out or do anything," he says.
Shanon became worried. Any time their extended family tried to get together for a birthday party or holiday, Justin refused to go.
"He would have all kinds of excuses," she says. "He was really depressed. He was down on himself real bad."
Most concerning was his lack of time with Autumn, who got upset when her father wouldn't play outside.
"She loves him. He's her hero," Shanon says. "I [knew] why he was feeling that way... but there was nothing I could do about it."
Small steps for a big change
Since his diagnosis with type 2 diabetes in March -- and his pinky promise to Autumn -- Justin has lost more than 65 pounds. He walks around his neighborhood whenever he can and makes sure to do the little things like taking the stairs and returning his shopping cart. The family is also eating better with more vegetables, leaner meats and less sugar. For example, at dinner Justin usually eats a spinach leaf salad with tuna or baked chicken with broccoli. Shanon joins in; she's lost 20 pounds in less than six months on the new diet.
The family recently took a trip to the Oklahoma State Fair and walked around all day, something they never would have done last year.
"I no longer lay in bed thinking that my weight is just too much to overcome," Justin says. "I enjoy being out with family and friends and having a good time."
He only weighs in once a month to avoid becoming obsessed with the numbers on the scale. He hopes to get down to 200 pounds, a weight at which his doctor said he would probably be able to go off his medications.
"I just want him to be healthy and happy. I don't care what weight he's at," Shanon says.
The couple raised more than $7,500 for the American Diabetes Association's "Step Out: Walk to Stop Diabetes" in Oklahoma City on September 14. Their team wore matching black T-shirts with a new slogan: "Do you pinky promise?"
On his website Justin has included a section where he asks readers to make their own promises -- to themselves or to their family -- to get healthy.
"[Autumn] taught me that you can do anything you put your mind to," he says. "It doesn't take a big giant step.It takes doing a little something."
Justin still has a long way to go in his weight loss journey. But rest assured that he'll keep going -- you just don't break a pinky promise, especially one made to your little girl.