"I wanted to come back from failure and be stronger the next time," he said.
"I wanted to add the extra pack to represent their burden that they carry," he said. "They don't take that pack off. They carry it with them for the rest of their lives."
McCastle, an endurance athlete and command fitness leader who's based in Whidbey Island, Washington, came up with the idea for his pullup challenge last year, when he was recovering from a torn anterior cruciate ligament and meniscus he sustained in 2012 at Basic Underwater Demolition/SEAL training prep school in Great Lakes, Illinois.
His lower body was still healing, he said, but pullups were doable. He gave himself eight weeks to train and was confident going into the event, which he staged at a public park near Naval Air Station Whidbey Island, but he had to give up 19½ hours in.
This time he took about four months to train, following two more strength challenges: a 29,029-foot rope climb — the elevation of Mount Everest — by scaling a 20-foot rope more than a thousand times, and a 13-mile, 250-pound tire flip last December, to raise money for the Michael J. Fox Foundation for Parkinson's Research and the Wounded Warrior Project, respectively.
But he wanted to make sure he finished the pullups, he said.
"This became a personal event when I didn't succeed at it," he said.
While training, McCastle worked his way from 50 pullups a day in June 2014 to 1,500 daily by mid-July 2014, putting him at six per minute at his fastest.
During both events, he slowed things down to about four and a half reps per minute to start, then slowed down in the later hours for an overall average
The key this time, he said, was to get back to that pace, but more importantly work on his cardio endurance. Rather than focus on pullup reps, he said, he worked in plenty of long runs and sprints to make sure his body was using oxygen as efficiently as possible.
McCastle had to take several days of leave last year after his hospital stay, but he said his command remained supportive and trusting that he wouldn't hurt himself this time.
A friend filmed McCastle during the event for submission to Guinness World Records.
With three feats under his belt, McCastle is continuing on with his "Twelve Labors Project," a series of events aimed at physically challenging himself and raising money for charity. He's mulling ideas, he said, including more strength feats and potentially a water-based event.
"I'm kind of superstitious," he said. "I don't really want to put it out in the universe until I recover and evaluate where my body's at."
Though he's taking a few days of leave to recover, he was in good spirits.
"I feel like a million bucks," he joked. "And I feel like I got hit by a train."
Meghann Myers is the Pentagon bureau chief at Military Times. She covers operations, policy, personnel, leadership and other issues affecting service members. Follow on Twitter @Meghann_MT