Billionaire Jared Isaacman spent 2½ years training for a historic 10-minute spacewalk — while simultaneously running a nearly $8 billion company.

Isaacman, 41, financed and commanded a SpaceX mission last month that featured the first-ever all-civilian spacewalk, during which he personally floated outside a Dragon space capsule. He's also the CEO of payment processing company Shift4 Payments, which he founded in his parents' basement in 1999 and grew to a market cap of $7.86 billion, as of Thursday morning.

"I've had two parallel careers for my entire adult life, between what I do in my day job at Shift4 and my aerospace career," Isaacman tells CNBC Make It, adding: "You don't sleep a lot."

As a CEO and father of two, Isaacman doesn't have a ton of free time — and his duties at Shift4 require "the vast majority of my time and energy," he says. Still, over the past couple of years, he spent most of his down time preparing for a five-day trip that took him and his three crew members farther from Earth than any humans have traveled in five decades.

Isaacman, who has an estimated net worth of $1.5 billion, says he didn't take a leave of absence during those two-plus years of training. He "stayed very involved" in Shift4's daily operations throughout that time, he adds — a period that saw the company make multiple large acquisitions, including buying point-of-sale systems company Revel Systems for $250 million in May.

"It is a balance," says Isaacman. "I basically just ate into a lot of my sleep. So there's a lot of nights and weekends that you spend at SpaceX ... The only time I was legitimately out of contact was about five or six days [during the mission]."

The trip was Isaacman's second space venture: In 2021, he financed and led the SpaceX team that became the world's first all-civilian crew to reach orbit. This time around, his training was more intensive — involving preparation and contingency work for deep-space issues like high radiation, micrometeoroids and orbital debris.

Isaacman and the crew also tested out various iterations of SpaceX's EVA spacesuits — "different joints, different rotators, different stitching for the fingers," he says — and practiced the spacewalk itself, ahead of actually doing it nearly 800 miles above Earth's surface.

"What if you can't re-pressurize the vehicle? How do you come home in those circumstances?" says Isaacman. "There's always things that pop up. I don't know if [there's] been a perfect human spaceflight mission in the last 60 years."

Once in space, the crew worked around the clock, conducting dozens of research experiments in five days while monitoring the capsule and its systems. Isaacman only got "a handful of hours" of sleep over the entire five-day mission, he says, calling it "a massive sleep deprivation event" for him and his crewmates.

Isaacman's day job was the farthest thing from his mind while he was in space, he says. Shortly after returning to solid ground, he was back at work.

"Within 48 hours of coming back to Earth, I made it to an investor event in [Los Angeles]," says Isaacman.

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