There were no more Earth-based challenges remaining for Rudy Smith. So that left just one way for him to go: up.

That’s literally what the 73-year-old retired Westwego postmaster will be doing Wednesday when he participates in the 39th Annual Empire State Building Run-Up.

The event begins in the main lobby of the iconic skyscraper and ends on the 86th floor observation deck — a total of 1,576 steps.

Smith, now living in Gretna, has run marathons on all seven continents, including Antarctica, and is a world-class duathlete and triathlete in his age group. For him, the Empire State Building is just another challenge — but with a bit of fantasy fulfillment thrown in.

“I’ve always loved King Kong,” he said. “I won’t get to do it like he did, but this is the next-best thing.”

Just to be able to participate in the run-up, Smith had to be one of 75 winners of a lottery drawing that annually attracts thousands of entries. Smith is the oldest of that group.

The lottery winners follow the elite competitors’ race, which also has charity and celebrity divisions.

The men’s record for the event is 9 minutes, 33 seconds. Smith said he hopes to break a half-hour.

He recently trained for the event by going up seven floors of steps in the West Jefferson General Hospital parking garage, followed by doing 1,800 steps on a Stairmaster at the appropriately named Manhattan Fitness Center.

The chief lesson learned: Lift your feet.

“If you let your feet drag, you’re going to trip,” Smith said. “It also takes a big toll on your legs, and my calves were killing me the next day.

“When you’re running, you can speed up or back up. Going up stairs, there’s no relief for your legs unless you stop, which I don’t intend to do.”

The 75 runners in Smith’s heat will start in five-second intervals, and, because of his age, Smith will be at the back of the pack.

“I’m going to make it,” he said. “I just want to do it in a decent time.”

But that doesn’t mean Smith will accept finishing where he starts.

“I’m going to pass somebody; I can assure you of that,” he said.

Among other things, it’s that competitive nature that keeps Smith active.

He was the bronze medalist in his age group at the 2015 World Duathlon Championships in Ottawa, Ontario, and, more recently came in second in the Dustbuster Duathlon in Shreveport and fifth at the Disneyworld Marathon, an event he has run for the past 23 years.

Later this year, Smith will compete in the U.S. Duathlon Championships in Bend, Oregon, while staying in training by running local events, such as the Jazz Half Marathon and Crescent City Classic, along with weekly workouts with the West Bank Track Club.

“When I’m in a race, I’m out to win my age group,” he said. “It’s not the medals, but the only way you know you’re getting better is being able to compare yourself to others.”

And then there’s the travel.

Smith and his wife, Deanna, who have been married for 48 years, have seen the world starting with both of them running in the 1996 Paris Marathon.

They’ve also visited London, the Great Wall of China, Australia and Mount Kilimanjaro, to name a few sites.

And at each stop, they’ve also become tourists — cruising down the Yangtze River to Shanghai, diving in the Great Barrier Reef and camping for a week on the African plains, among other activities.

In New York, where both Rudy and Deanna have run the New York Marathon several times, they’ll take in a Broadway show or a concert at Lincoln Center.

“Running has enriched our marriage a lot,” said Deanna Smith, who no longer runs marathons due to knee problems but still manages to run and walk in half-marathons and shorter races. “We’ve scheduled our vacations around runs, and that’s kept us together more.

“I try to go wherever he goes. Whenever I can’t run, I’m his biggest cheerleader.”

But Smith’s desire to keep fit, and to be an example to others, is his top motivator.

A high school hurler, Smith didn’t take up serious running until he was nearly 40. Trim but not skinny at 5-foot-11 and 165 pounds, Smith stresses healthy eating — more vegetables and less meat, cutting back on bread and avoiding sugars — along with staying active as the key.

“Walking is a good place to start,” he said. “You can swim, play tennis or do line dancing — anything that gets you moving and makes you sweat. I see people who are sedentary, and it makes me sad because they are just short-changing their lives.”

Smith isn’t all about running.

Since his retirement after 42 years with the postal service, he has become active in the Belle Chasse Rotary Club, the Gretna Civic Association, the Gretna Historical District and Boy Scout Troop 1101, of which he is assistant scoutmaster.

“I’ve made a commitment to be a good citizen,” he said.

Smith concedes he’s been fortunate to have no unavoidable health issues. As he gets older, he said he finds the need to stretch more as his main concession to aging.

Still, Smith knows that, eventually, the time will come when he won’t be able to compete at his current level.

That why there are still a couple of items on his bucket list, like the Ocean-to-Ocean ultra-marathon in South Africa and the North Pole Marathon, although the $20,000 entry fee for that one is proving daunting for a senior citizen on a fixed income and will take sponsorship help to make possible.

The Empire State Building Run-Up is Smith’s first such attempt at that kind of competition.

And it will also be his last.

“I mean, there’s only one Empire State Building,” he said. “Once you’ve done it, why bother to go anywhere else?”

After all, it was good enough for King Kong.