For Gary Marino, the turning point came on an idyllic vacation...that wasn't. In Maui with his wife, Julie, in 2000, Marino, then 397 lbs., "snorkled, and the equipment didn't fit," he says. "The furniture was wicker, and I kept thinking, 'I wonder if this is going to collapse?'" Recalls Julie: "I love to explore, but he wouldn't walk 10 feet."

He's making up for it: 130 lbs. thinner, thanks to a low-carb diet and exercise (key ingredient: walking), Marino has set off on a Million Calorie March from Jacksonville, Fla., to Boston. Along with losing another 70 lbs., he hopes the walk—80 days, 1,200 miles—will raise awareness about obesity as well as cash for Generation Excel, his fat-fighting foundation. "There are walks, jogs for all kinds of diseases," the 38-year-old entertainment entrepreneur says, "but not much is being done for obesity"—the No. 2 preventable killer. To date Marino has received $115,000 in corporate and individual donations.

Overweight since childhood in Medford, Mass., Marino has a less tangible goal as well. "I want people who've never struggled with obesity," he says, "to have compassion for those who have."