For Mr. Duffy, going vegan in a firehouse was a challenge. “Guys at work joke with me, they’re like, ‘You’re going to die at 45, watch, and we’re all going to laugh at you,’” he said. “It’s a very opinionated group of guys. They don’t like too much change. They like good heavy meals,” like chicken Parmesan, steak or meatballs, he said. “I’ll stack my locker with cans of beans and brown rice.”

Mr. Duffy, who shares vegan recipes and training tips on his Instagram account, @fire_and_ironman, said that, joking aside, many firefighters are truly curious about his regimen. “I’ll make quinoa and beans with soft-roasted veggies, carrots, squash, sweet potatoes,” he said. “My stuff comes out and it’s like, ‘What is that? Let me try that,’” He’s even managed to convert two other firefighters, and his mother, to his plant-based diet.

For both men, the appeal of veganism is not political or ethical, but about reaching higher performance levels as athletes. “You can get into the animal-rights thing — that comes with the territory. The environment, it’s helpful for that as well,” Mr. Duffy said. “But I’m just like, at the end of the day, I could kick my 22-year-old self’s butt!”

Back in the pain cave, the duo worked through a relatively light series of exercises simultaneously meant to help Mr. Quinn recover from a recent back injury and to ease Mr. Duffy back into triathlon shape after a six-week vacation in Asia. In between push-ups and situps, Mr. Duffy recalled the first time he noticed that veganism had affected his job as a firefighter. A year after he’d gone vegan, he said, his company responded to a fire at a split-level ranch home.