Paul Ryan said it was the honour of his lifetime to be the newly minted vice presidential candidate, veering from "surreal to the real" the moment Mitt Romney asked him to be his running mate.

"I was very humbled. It was the biggest honour I've ever been given in my life," Ryan told reporters aboard Romney's campaign plane as they flew from the Washington suburbs to Charlotte, North Carolina as part of a bus tour through crucial swing states.

"We're going to win this campaign," an exuberant Ryan said, with Romney at his side after a full day of campaigning across the state of Virginia. "We've got the wind behind us. I'm really excited about this race."

Romney shocked much of the political world when he tapped Ryan, the 42-year-old chairperson of the House Budget Committee, to be his number two in the battle to unseat President Barack Obama in November.

The Wisconsin congressman is known for his level-headed composure, but even he admitted to feeling an extraordinary sensation when Romney told Ryan he wanted him on the ticket.

"It's gone from the surreal to the real," Ryan said.

On the plane, he was warm and relaxed, but didn't hesitate to get down to business.

"I love this country dearly. And I feel like we have an opportunity to fix things once and for all. I feel really strongly about that," Ryan said.

"I'm excited about this, and I've spent most of my adult life fighting for these ideas."

Romney too appeared looser than his rather stiff public image often suggests, saying he was impressed by the "energy" of supporters in Virginia.

And the former governor of Massachusetts sounded thrilled to finally have an official running mate who can double the fight the Republican camp can bring to Obama and Vice President Joe Biden.

"It's now two on two, instead of two on one, that's good," Romney said.