For Rudy Smith, there were no more Earth-based challenges remaining. So that left just one way for him to go — up.

That’s literally what the 73-year-old retired Westwego postmaster did last Wednesday when he participated in the 39th Empire State Building Run-Up.

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

And Smith — a Gretna resident who has run marathons on all seven continents, including Antarctica, and is a world-class age group duathlete and triathlete — hit every step, finishing in 22 minutes, 40 seconds — more than seven minutes faster than he anticipated.

“I’ve always loved King Kong,” Smith said. “I didn’t get to do it like he did, but this was 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 gets thousands of entries. Smith was the second-oldest male runner.

He won his age group and finished 151st of 211 total runners in the event, which also has elite, charity and celebrity divisions.

“I did way better than I thought,” Smith said. “About half-way up, I was feeling pretty good, but it got tough after that. But it still turned out real good.”

Smith said he ran up the first three flights, but after that, like all but the elite competitors who finish in about 10 minutes, it was more of brisk climb, aided by the handrails.

The race presented a couple of unexpected obstacles — switchbacks between floors meant negotiating turns and dust — which made the cool air on the observation deck feel all that much better when he reached it.

The 75 runners in Smith’s heat started in five-second intervals, and, because of his age, Smith was near the back of the pack.

While he was passed by some, he also got ahead of the woman who started before him.

“I was going to pass somebody,” he said. “I could have assured you of that.”

It’s that competitive nature, among other things, that keeps Smith active.

He was the bronze medalist in his age group at the 2015 World Duathlon Championships in Ottawa, Ontario, and was fifth at the Disney World Marathon, an event he has run for the past 23 years.

“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 wife Deanna have seen the world, starting in 1996 when both ran in the Paris Marathon. They’ve also done London, the Great Wall of China, Australia and Mount Kilimanjaro, to name a few.

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

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 sugar — 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 your sweat.”

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

Smith knows that eventually there will come a time when he won’t be able to compete at the level he’s at now.

That’s why there’s still a couple of items on his bucket list, like the Old Mutual Two Oceans 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.

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

And it will 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, even if it didn’t work out too well for him.