When Pat Tiernan stood at the start line at last weekend's Houston Marathon, he had one time running through his head — 2:08:10.

The Olympic standard time.

It would require him to run the 42.195-kilometre course at a pace of 3 minutes and 2 seconds per km.

"It's a pretty daunting task ... it's only 19 seconds between that and Deek's (Robert de Castella) old Australian record," Tiernan said.

By the time he crossed the finish line, he had not only achieved his goal — he also became the third Australian to ever finish a marathon in "the sevens", in a time of 2:07:45.

"I don't think it's really sunk in," he said after running a time that should see him selected for the Paris Olympics in August.

"But I prepared for it and felt more than capable when I got to the start line ... and let the racing instincts take over from there.

Tiernan is no stranger to the big stage.

The two-time Olympian won the hearts of Australian sports fans when he collapsed with exhaustion before getting up to finish the 10,000m final at the Tokyo Olympics in a personal best time.

The 29-year-old from Toowoomba hung up his track spikes after the Games in 2021, setting his sights on an Olympic event steeped in history.

The Tokyo Games were delayed a year because of the COVID pandemic, shortening preparations for the next event by 12 months.

"My high school coach always had a feeling I'd be a marathon runner," Tiernan said.

"But I think up until the last couple of years, 42 kilometres just seemed like a really long way.

"It was just a matter of when I felt I was ready to do it, and that time is now."

The Paris marathon course has four times the elevation of the equivalent route in Tokyo.

But Tiernan, who grew up on the hilly streets of Toowoomba, is not daunted.

"I definitely think it could be a course that I can run well on; it's just a matter of, once again, learning this new event," he said.

"I can use the hills to my advantage, but I've still got to be able to tackle the distance itself."

The reverence with which Tiernan speaks of the marathon distance has impressed one of Australia's greats.

"It's a long way, and a hell of a run ... and it's a really healthy that thing Pat respects the marathon," Robert de Castella said.

"Because if you don't respect the marathon, it'll bite you on the bum."

De Castella cheered Tiernan from his home in Canberra, and was excited when he crossed the finish line, even though the time he ran eclipsed his own best.

"I'm absolutely thrilled that we're starting to get back to sub-2:08 marathon runners representing Australia, and I think that's just going to inspire more people," he said.

Only three Australians have gone under 2:08 — Brett Robinson, who set a new national record in 2022 (2:07:31), Tiernan (2:07:45), and De Castella (2:07:51).

"You've obviously got to have pretty good endurance for the 42km, but you've [also] got to be really strong.

"The strength comes through in that last 5 or 10 kilometres, and with these guys coming off the back of really good track performances, they have that endurance fitness, and they've got the legs to really get up there and be competitive and aggressive over that last five or 10 kilometres.

"That's what I'm really excited to see; runners like Brett and Pat stepping up, and really taking it to the rest of the world."

De Castella ran his best time in 1986, to win the Boston Marathon.

It took 36 years for an Australian to run faster.

But De Castella is excited the two runners that have eclipsed his record have done it recently and should both be on the start line at the next Olympic games.

"I think it augurs well for an exciting future," he said.

"With Pat and Brett up there, hopefully they can really push each other."

Tiernan is humble in response to the praise.

"We're just the sideshow," he said.

"Our women are crushing it; I think there's about five or six of them that have the (Olympic) standard.