Documentary filmmakers live to immerse themselves in a subject, to seek to erase the line between witnessing and experiencing, to turn straightforward, objective reality into swirling cinema verite.

So when Reno, Nev., documentarian JB Benna ambitiously sought to capture the feeling of what it takes to win the Western States 100 Mile Endurance Run — arguably the world’s most famous ultramarathon, contested each June from Squaw Valley to Auburn, Calif. — it meant doing something that D.A. Pennebaker or Michael Moore could neither fathom nor physically perform.

He ran right along with the leaders, pointing a 10-pound camera with boom mic at them.

Not the entire 100.2 miles, of course. But Benna, an ultrarunner himself, figures he logged about 34 miles, up and down rock-jagged canyons, through creeks and snowcapped peaks, following the top four runners during the 2010 Western States. He was supported by eight other cameramen, leapfrogging aid stations along the course.

But much of the footage in “Unbreakable: The Western States 100,” which will be shown in screenings starting this week and be available on DVD next month, is shot by Benna, who gamely tried to maintain the blistering pace of winner Geoff Roes, who set a course record that year.

That’s not something they teach at the USC School of Cinema-Television, where Benna, 32, studied before working for Hollywood studios and eventually starting his own production company, Journeyfilm.

“Though I think there’s a lot more shakiness in shots running with the camera, you get to be with the runners longer and get to feel the speed and the terrain of the race, rather than just seeing a guy run past the camera, then cut to another guy running past the camera and so on,” Benna said.

“My wife mentioned, the first time she watched it, she said, ‘You have these [runners] talking about the thing they’re actually doing in the moment rather than [after the fact] and showing some random clip,’ which seems to be a big thing in documentary filmmaking.”

Benna’s documentary is not the first centered on Western States. In the 1980s, television network ABC documented the race on two occasions, and PBS also featured the “human drama” of men and women spending up to 30 hours on the trails.

Those earlier attempts captured the history of the event and focused on human interest stories, and Benna applauds their work. But he wanted to try something altogether different: give the audience an elite runner’s view of the race.

That meant prolonged stretches where he ran alongside the sport’s top runners — Roes, Anton Krupicka, Kilian Jornet and Hal Koerner — pointing the lens right at them.

“I think they actually enjoyed seeing me along the way because I was the person they saw the most throughout the day, even more than their crews,” Benna said. “It was kind of like an unofficial pacer. They’d vent to me, which was good for the film because I could get how they were feeling [in real time], talking through some issues.

“Geoff would say things like, ‘I can’t run uphills [well] right now,’ or ‘My stomach’s falling apart. I gotta do some work to turn this around.’ All of that ends up in the film.”

Benna picked a good year for the project. The 2010 race featured the favorite Roes, an Alaskan who had never lost a 100-mile race, against Coloradan Krupicka and Spaniard Jornet, neither of whom had ever lost a race. The fourth runner was defending Western States champion Koerner, a close friend of Benna’s.

Benna himself ran Western States in 2009, on a lark filming himself and others while still trying to compete. What footage he got that year turned out to be disappointing. So he knew that he would need logistical help from other cameramen as well as his own work among the leaders to adequately do the race justice.

“It’s nearly impossible to get enough shots to put together a story if you follow more than a small group of people,” he said. “I had eight cameras out there and I still feel like I’m using every single shot we got.”

To order the DVD or find theater screenings of “Unbreakable: The Western States 100,” go to www.ws100film.com.