Depicting a trek along one of the longest routes in the US - the 2,180-mile Appalachian Trail - would be taxing enough for a sprightly young actor, let alone a couple of septuagenarians.

But Hollywood veterans Robert Redford and Nick Nolte did just that for upcoming comedy drama A Walk In The Woods, based on author Bill Bryson's 1998 book about his attempt to complete the famous route from Georgia to Maine.

"You'd go up 100ft and they'd say, 'OK cut', then you'd do it again - and you'd do that seven or eight times. It was very physical, as it turns out," says 79-year-old Redford, who retains the rugged good looks that helped him find fame.

"There's the obligation you feel as an actor to completely embody the role. You can't just do it with your face; you've got to do it with your whole body."

The Butch Cassidy And The Sundance Kid star could relate to the sense of spontaneity and adventure that led Bryson to undertake the challenge. As a 16-year-old, he and his brother decided to climb Mount San Jacinto, a 10,000ft peak in their native California.

"I said, 'Let's just go to the top'. So we started to go for the top, where there was snow, as the sun was going down. It was stupid, we should have gone back right away, but we went up there. It turned dark, it was cold and the ice looked like rock; it suddenly became very, very dangerous. But I just went without thinking it through," he recalls.

Redford, who emerged from the experience unscathed, admits he's "not a person that can plan very well".

"[Sometimes] you find yourself a new territory that's exciting and you say, 'That was worth taking a risk', and there are other times when it isn't. But I think there's always been an impulse to go into uncharted territory and see what happens," he adds.

"[Bryson] didn't quite know what he was doing; he was innocent and he had this impulse to do something."

Redford plays the hugely popular writer in the film, who undertook the trip after two decades living in the UK, while former hellraiser Nolte, 74, stars as Katz, the recovering alcoholic buddy who accompanies him.

The movie's been a long time in the making. Redford - who also produced the film - discovered the book back in 2002 while on holiday with his second wife Sibylle Szaggars, and soon set about trying to secure the rights.

"Bryson is such an interesting writer, and he's got a wonderful sense of humour - it's witty, it's sardonic, all elements I like," he says. "It had a wonderful friendship; friendship lost and then regained. So you put all these things together and I thought, 'This could be a really good film'."

The role of Katz was originally intended for his Butch Cassidy co-star Paul Newman (who died in 2008, aged 83), but his age and failing health became an issue early on, so Redford got in touch with Nolte. He'd previously directed and appeared alongside the Down And Out In Beverly Hills star in 2012 thriller The Company You Keep.

"Nick and I are the same age, we were both a mess when we were young, I straightened out somewhat, he straightened out a little bit," Redford says with a laugh.

"Working with him, there was something so natural, it just fell into place. I felt a synchronicity with him."

Bryson's wife, meanwhile, is played by Emma Thompson - a "very funny, very witty, wonderful actress, and she has a great big heart".

During his more than half a century in the business, Redford has tended to veer away from mainstream fare, although his CV includes plenty of hits.

After taking acting classes in New York, the former college dropout began landing TV and stage roles in the late 1950s, and went on to become one of Hollywood's best-known names, appearing in 1973 films The Way We Were and The Sting - which saw him nominated for an Oscar - and The Great Gatsby in 1974 .

He set up the Sundance Film Festival, celebrating independent film, in the late Seventies, and his first foray into film directing, with 1980 Donald Sutherland drama Ordinary People, saw him win an Oscar for best director.

Even as a well-respected veteran of the industry, however, Redford admits he still struggles to get movies green-lit.

"The films I'm more inclined to like are the smaller, more humanistic films that may not bring such a big return, so therefore they're harder to get made," says the actor, who took a break from indie titles to appear in last year's blockbuster Captain America: The Winter Soldier. It's always been hard and it's going to stay hard.

"[A Walk In The Woods] was made for just under eight million dollars, very low budget," he adds. "The climate's changed. The high-paying films - films like what Tom Cruise is doing - those are big action films that pay very well on their return, so Hollywood's going to go there and put that big money into it."

So what keeps him going, after all these years, in what can be a cut-throat business? It doesn't take Redford long to reply.

"Breaking new ground, creating something new and fresh," he says. "That's exciting."

:: A Walk In The Woods is released on Friday, September 18