By Teri Berg and Paul Suwan

One day when the elevator was out in his apartment building, John Korff had to make several trips up and down the stairs to retrieve his packages.

The huffing and puffing he did changed his life.

“I was sweating and my heart was beating, and I thought, ‘Wow, what a great workout!’” said Korff, 54, who competed in his eighth Empire State Building Run-Up on Tuesday.

The New York businessman had given up distance running – the toll it had taken on his back forced him to have surgery. Afterward, he found he hated going to the gym.

On the heels of his revelation, Korff found that running the stairs is one of the best cardio workouts he can get.

He also likes it because it’s time-efficient, free and takes him to new heights in adventure.

Korff starts training for the ESB Run-Up six months prior to the February event. In August, his goal is simply to begin building strength and endurance.

He often runs in his office building in Midtown, which has 25 floors, but he also tries out a variety of other buildings around the city, looking for progressively higher step counts, longer flights of stairs and more floors. One of his favorite haunts has 40 floors and 600 steps.

Korff trains three or four days a week. At first his pace is casual and he will spend 40 to 55 minutes running up 25 floors four times.

“When I’m training, I only climb — never descend,” Korff said. “Going down the stairs puts too much pressure on the knees. So once I get to the top floor, I take the elevator down and start up again.”

After two months, Korff adds both speed and more floors – now he’s ready to push himself to climbing that 25-floor staircase six to eight times per workout.

In the final six weeks before the race, Korff will sprint up 40 floors up to six times, which is nearly the equivalent of running up the Empire State Building twice.

Korff noted that it’s hard for him to find new buildings to train in these days — more buildings have security staffs and lock up early because of the effects of 9/11.

“I probably get thrown out of 10 buildings a year,” he said.

He recounted a time in Zimbabwe when, while accompanying the U.S. Davis Cup tennis team, he was chased through a building by a security guard who was toting a machine gun.

“His English wasn’t good, so he couldn’t understand what I was trying to tell him,” Korff said. “I just had to outrun him, which was a cinch.”

Once, while running the stairs in Trump Tower, with permission from Trump himself, another security guard gave chase. When the hefty gentleman fell down from exhaustion, Korff stopped and went back to give the guy a hand.

The New Jersey native, who has run over 140 marathons and seven 100-mile races, said his training this year was not as intense as in past years.

“It’s hard to spend all my free time training and have time for my wife and son, too,” he said.

But for Korff, having limited time just makes him more creative.

When he travels to Hawaii for business this year, he already knows what hotel he’ll stay in – the Sheraton Waikiki.

Why? Because that particular Honolulu-based hotel has 31 stories and 423 steps.