"For me it's not so much a sporting challenge, and it's not a financial challenge, it's not any of those things," he said.
"I came back as a volunteer and so I'm here for the love of the bike and the passion of the cause."
However, Armstrong says he is fitter now than when he was at the height of his success.
"I've prepared much harder this series of months than I ever would have in the past," he added.
"The tests that we do on the bike, or on the road, or in the lab indicate that my January fitness is much better than it ever was during the years when I was winning the Tour.
"But that doesn't mean anything until you get into the race.
"It's not just about the physical conditioning, it's about what it feels like when 200 guys go down the road and go around dangerous corners at 40-50 miles per hour.
"That's a dynamic that you can't simulate in training. These are the things that most people look at and think we're nuts to do that. I've got to get back into that space."
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