As a teen growing up in the small Hudson Valley town of Saugerties, N.Y., Jimmy Fallon often begged his parents for permission to stay up late and catch a few minutes of Johnny Carson on the "The Tonight Show." And though he got a kick out of Carson, he never had the audacity to picture himself as the host of NBC's storied late-night institution.

"A lot of people are asking, 'Is this your dream job?' It wasn't at all my dream job, because I didn't know that this could be a job that you could dream about," says Fallon, who notes that more people have walked on the moon than have hosted "The Tonight Show."

Jimmy Fallon, who is replacing fellow Saturday Night Live alum Conan O’Brien as the host of NBC’s "Late Night" talk show, now known
Jimmy Fallon, who is replacing fellow Saturday Night Live alum Conan O'Brien as the host of NBC's "Late Night" talk show, now known as "Late Night with Jimmy Fallon," poses for a photograph in his Rockefeller Center office, Monday, Feb. 23, 2009, in New York. (AP Photo/Diane Bondareff) ( Diane Bondareff )

On Monday, the impossible dream becomes reality when Fallon, 39, takes over late night's top-rated program from Jay Leno, who capped a 22-year run earlier this month. He will be only the sixth host since "Tonight" began in 1954, joining a select group that includes Steve Allen, Jack Paar, Carson and -- very briefly -- Conan O'Brien.

Of course, the late-night turf he inherits is vastly different from what it was under Carson, or even in Leno's early years. As a writer for "Variety" observed, what was once a kingdom is now divided into fiefdoms, with competition in the form of everyone from David Letterman and Jimmy Kimmel to Jon Stewart and Stephen Colbert.

Given that fragmentation, ratings have steadily declined. Leno's "Tonight Show" averaged 3.7 million viewers an episode in 2013, down from more than 6 million at its ratings peak in 1997, and a fraction of Carson's high point of 15 million. But even in 2014, plenty of prestige is packed into "The Tonight Show," which looms large in American pop-cultural history. It also still makes lots of money for NBC -- reportedly delivering more than $125 million in ad revenue last year.

So the expectations -- and the pressure -- remain high.

"It's surreal, and it's an honor," Fallon told reporters at the recent TV critics press tour. "I don't want to let anybody down. I want (viewers) to know that I'm going to work as hard as I can. I'm a big fan of television. I know how much the show means."

Some industry observers have questioned the move's timing. Leno, after all, was still a strong No. 1, and Fallon, a former "Saturday Night Live" standout, had been faring well on NBC's "Late Night" at 12:35 a.m. However, advertisers crave young viewers, which partially explains why NBC wanted to pass the torch to a younger, more digitally savvy host. Kimmel's fast rise on ABC also was a factor.

Jimmy Fallon, who is replacing fellow Saturday Night Live alum Conan O’Brien as the host of NBC’s "Late Night" talk show, now known
Jimmy Fallon, who is replacing fellow Saturday Night Live alum Conan O'Brien as the host of NBC's "Late Night" talk show, now known as "Late Night with Jimmy Fallon," poses for a photograph in his Rockefeller Center office, Monday, Feb. 23, 2009, in New York. (AP Photo/Diane Bondareff) ( Diane Bondareff )

At least this transition lacks the messiness of the one in 2009-10, when Leno was forced out of his "Tonight" job in favor of O'Brien. Leno, who openly griped about the move, was off the show for nine months, during which he faltered as host of a prime-time series and the "Tonight" ratings declined. One of the biggest debacles in TV history culminated with O'Brien exiting and Leno regaining his throne.

Now, the top ranking in late night is Fallon's to lose. He figures to open strongly, thanks to the promotional oomph of NBC's Winter Olympics coverage. When the Games end, the real competition begins.

Immediately, however, viewers will see changes that extend far beyond the man at the desk. For starters, the "Tonight Show" has relocated from Burbank to Fallon's home base of New York for the first time since Carson moved the show to the West Coast in 1972. It will be taped on a sparkling new set in Rockefeller Center's Studio 6B, where Carson and Paar once presided.

"I just think it's the perfect place, where it should be," Fallon says of the move. "I think of the lights and Times Square and Broadway and night life and the excitement and the glitz and glamour of all that is the 'Tonight Show.' "

It wasn't that long ago that Fallon seemed an unlikely choice to carry the "Tonight Show" franchise forward. But he honed his interviewing skills during five years on "Late Night" and quickly won viewers over with his sweet nature and geeky energy.

He also displayed an eagerness to vacate his desk and get silly. Fallon has performed musical parodies with Bruce Springsteen and Neil Young, "slow jammed" the news with President Barack Obama, danced with first lady Michelle Obama and done spot-on imitations of Barry Gibb. Along the way, he and his writers built a show tailored for the new-media world, teeming with Internet-friendly sketches that tend to go viral.

But will it play to the core "Tonight" audience? Leno's ratings success was largely attributed to his broad-based humor and general appeal to middle America. Fallon's approach might be different, but his supporters say he has the charisma to reach the masses. Josh Lieb, a producer for "Tonight," calls Fallon "the least exclusive comedian I know." And in a recent "Vanity Fair" piece, Tina Fey talked of how her former "SNL" cast mate established "Late Night" as an "uncommonly warm, welcoming show."

Fallon insists the migration to a new time slot will not change him. And he's pretty sure his predecessors would approve.

"I wish Steve Allen and Johnny Carson were still around to see what we're going to do with the show, because I think when they invented ("Tonight"), it was all about being fun and silly and goofy," he says. "Watch us, and you get a good laugh, and you go to bed with a smile on your face."

Follow Chuck Barney at Twitter.com/chuckbarney and Facebook.com/bayareanewsgroup.chuckbarney.

"the Tonight show starring jimmy fallon'

When: Midnight Monday
Where: NBC
First guests: Will Smith, U2