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Marathons, in general, are unpredictable. The course layout, the weather and the previous night’s dinner can combine to punish even the savviest of elite runners and work in favor of a plodder.

That notion hasn’t stopped Terrence Mahon, coach of U.S. marathon trials champion Ryan Hall, from thinking that the 25-year-old from Big Bear might win here if everything plays out right.

“We’re definitely throwing time out the window, per se, in terms of trying to run a fast time. I don’t think anyone is going to run fast times . . . except maybe the sprinters, just because of the conditions,” Mahon said of the haze, heat and humidity that have been prevalent in Beijing.

“The first thing we did was get his head off of time. But if you look at where his time is this year, he’s going to be one of the top 10 guys in terms of performance, and I think whenever you’re someone like that you definitely have a shot of getting on the podium.”

Several factors could work against Hall, a Stanford graduate who trains with the Running USA group in Mammoth Lakes.

First is that he’s young for an event that usually takes time to master. In addition, he has run only a few major marathons.

But if he lacks quantity on his marathon resume, he has made up for it with quality performances.

In 2007, he ran the fastest debut-marathon time by an American, finishing seventh in London in 2 hours 8 minutes 24 seconds. He followed that in November by breaking the Olympic trials marathon record on a tough course in New York with a winning time of 2:09:02.

This year Hall was fifth at London in 2:06:17, making him the second-fastest American ever and giving him the seventh-best time in the world this year.

He may be a long shot in Beijing on Aug. 24, but there’s no consensus favorite.

World record holder Haile Gebrselassie of Ethiopia, who has the world-leading time this year of 2:04:53, is skipping the marathon because he fears the pollution will aggravate his asthma. Martin Lel and Samuel Wanjiru of Kenya, who finished ahead of Hall in London, are expected to lead the field.

“If we look at these form charts and the history of everyone that’s been a favorite going into an Olympics or world championships in tough weather, those are the ones that don’t fare well,” Mahon said.

“The two world record holders at the last Olympics did not have their best days and that’s been seen time and time again.”

Mahon has prepared Hall for anything when his event begins Aug. 24.

“Being that this is his first Olympics and really he’s pretty new to the marathon, we’re looking to just kind of not make mistakes, and if that sets him up well for the last part of the race, then he’ll take a swing,” Mahon said.

“I think our biggest thing is taming the emotions, let him enjoy the experience. Getting him to run smart. And I think if he does that, then it wouldn’t be an unforeseen thing that he could get a medal.”

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helene.elliott@latimes.com