A version of this story appears in the Aug. 26–Sept. 2, 2019, issue of Sports Illustrated. For more great storytelling and in-depth analysis, subscribe to the magazine—and get up to 94% off the cover price. Click here for more.

Charles Tillman’s hands are rough and scratched. There are twin calluses at the base of his middle and ring fingers, like a pair of mountains rising from his palm. He paddles his left oar, over and over and over again, fighting a strong current that’s keeping him from reaching the harbor.

Tillman retired after a 13-season NFL career three years ago, and his trademark move, the Peanut Punch—a quick right jab he’d use to knock the ball away, helping him force 44 fumbles over his career—never gave him tough hands like these. On Sept. 2 (if weather allows), the former Bears cornerback will row 65 miles across Lake Michigan from St. Joseph, Mich., to Chicago’s 31st Street Harbor to raise money for pediatric cancer research and to support families with critically ill children. He’s teamed up with marketing exec and fellow philanthropist Jake Beckley to create Row4Kids, which combines their individual foundations to raise money.

Tillman was inspired to start his nonprofit, the Cornerstone Foundation, after his daughter Tiana had a life-saving heart transplant when she was five months old. They’re hoping to persuade 650 people to donate $1 for each of the 65 miles across Lake Michigan. “Yeah, my butt goes numb after an hour and a half, and I have blisters,” Tillman says of rowing. “But it’s nothing to what these kids go through when they have cancer.”

Beckley already rowed across Lake Michigan once by himself, and the trip was so difficult that he swore he’d never do it again. This time he and Tillman decided to build their own boat: Hope’s Journey, 22 feet long and five feet wide.

Rowing might look easy, but as Tillman quickly learned, it takes a lot of coordination. He worked with coaches at the Chicago Rowing Foundation to improve his form. “It’s more difficult than I thought,” he says. “I didn’t realize how in sync everything has to be. The sequence of putting it all together: Legs, core, arms, legs, core, arms. I’ll do it a thousand times before I get it right.”

He wakes up at 5 a.m. each morning to work out, rowing for two hours on his Concept2 Rower, which he moved to his patio to train in the heat and humidity. His training plan included many rows on Lake Michigan with Beckley, including two sessions of 10 to 15 hours.

During their 65-mile journey, Tillman will sit in the bow to set the pace, Beckley in the aft, to set the direction. (“I don’t want to be responsible for us rowing the right way,” Tillman says.) A chase boat will follow, stocked with tools they can’t fit in the rowboat: extra batteries, bolts, wrenches and, most important, a hot sandwich for Tillman. “I can only do snacks for so long,” he says.

The trip will probably take around 20 hours, and Tillman, who finds rowing to be boring, plans to occupy himself by watching all four Avengers movies in order. While training, he’s gone through all of Game of Thrones, Entourage, Star Wars and Lord of the Rings.

Tillman currently works for the FBI, but he isn’t allowed to discuss his job. All he can say is when he walks on shore after nearly a full day on the lake, he’ll need a day off work to recover.

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