Tim McGraw is a fitness buff — his routine is legendary — but the "Thought About You" singer has never explained why he chose to get fit a decade ago. Thank his daughter Gracie for helping him clean up.

The 52-year-old country star and actor reveals that in 2008, he felt he was at the peak of his career, with a long resume of country hits and a few roles in major motion pictures. But it was his appearance in a Vince Vaughn and Reese Witherspoon movie called Four Christmases that led to how he looks today. Speaking with Men's Health magazine, McGraw reveals that after his daughter watched it, the then pre-teen told her dad he looked a bit big. He weighed 215 pounds by then.

"I got out of it for a while,” McGraw says of a dedication to the gym. "I was in the prime of my career, and I wasn’t capitalizing on it."

To understand what that means, you have to understand what the physical transformation did for him. Quickly he eliminated drinking, fast food and truck stop dinners from his diet and began walking for 20 minutes each day. Then he started to run, and eventually built up a workout program the magazine describes as "vicious." It's all part of his new book Grit & Grace: Train the Mind, Train the Body, Own Your Life. 

"I use my whole body to sing—my legs, my butt," McGraw says. "And having more control over those things makes my voice stronger." In fact, soon after he changed his diet, he started to see tangible results on stage. He's a better, stronger performer now than he was back when he thought he was at the peak of his career.

With a Top 20 single and new music planned for 2020, it's not clear that his peak is behind him. As for Gracie, she's now 22 and off to chase her acting dreams in Los Angeles. McGraw fans got a taste of her talents in a recent car karaoke video shared on his social media pages.

WATCH: Tim McGraw and Gracie Sing Together!

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