Wasn't it enough that a sniper bullet missed, by centimeters, his head on an Easter morning in Fallujah? And wasn't it enough that weeks later he had to watch two of his fellow Marines get blown up when their Humvee ran over an IED, the same IED that his Humvee had avoided, seconds earlier? And wasn't it enough that he was nearly killed two months later when the Humvee he was in this time didn't miss an IED, and Keith Zeier—a perpetual-motion 20-year-old—was left with a left leg with virtually no sensation from knee to hip? Wasn't that enough anguish and pain?

No, Keith Zeier, will tell you, that wasn't enough. "Survivor's guilt," he says. "I'm alive and a bunch of my friends aren't. That keeps me motivated."

So last May, Zeier ran a 100-mile race from Key Largo to Key West, Florida. His mission: to raise money for the Special Operations Warrior Foundation, which provides financial assistance to wounded soldiers and college tuition to children of service men and women killed in action. Twenty-four hours aft er Zeier announced his attempt online, the foundation received $50,000 in pledges, the most ever in a one-day span. In total, Zeier, now 23, has raised $85,000, all with a leg he may have amputated one day.

Denise Zeier, Keith's mom, remembers her son was always in a rush as a kid. To soccer practice. To track meets. And to graduate from his Long Island high school so he could join the Marines. "He was 17, and he needed me to sign his papers," says Denise, a single mom. "Otherwise, the Marines won't take you until you're 18." But ever since September 11, 2001, all Keith wanted to be was a Marine. On that day, Lt. John Crisci of the New York City Fire Department, his best friend's dad and a father figure to Zeier, died in the World Trade Center. "Mr. Crisci would always be there for me," says Zeier, who's now hoping to join the FDNY. "He invited me wherever he and his sons went—soccer games, hockey games. After he died, I decided I wanted to be in the Special Operations. It was the least I could do."

Members of Special Operations are trained to undertake the most challenging operations. Zeier made Special Ops on his first attempt. But on July 17, 2006, less than five months into his Iraq deployment, his military career came to a crippling end with the roadside bombing. Shrapnel tore through his upper left thigh, severing nerves and causing massive bleeding. He had three operations in Fallujah, and then was transported to National Naval Medical Center in Bethesda, Maryland, for more surgery. A 12-inch scar is the external remnant of his troubles. Internally, "even though I have no sensation in my thigh," he says, "the rest of my leg hurts 24 hours a day."

Doctors told him to expect to use a cane the rest of his life. But after 20 months of physical therapy, Zeier attempted his 100-miler. By mile 75, following 21 hours of running, he lay on a Florida road being treated for dehydration and exhaustion. EMT officials recommended that he be taken to a hospital. Zeier wouldn't hear of it, signing a waiver refusing such treatment. Moments later, he started back up again. "Even if my legs weren't working," he recalls, "I was going to crawl to the finish." He finished the race in 31 hours, three minutes. "The mental aspects that got me through that day are the ones my buddies portray every day," he says. Then, remembering his buddies who will never return from Iraq, Zeier adds, "How could I quit on my friends who made the ultimate sacrifice?"