Asked whether Armstrong had followed Bush's rule against passing the President, Duffy replied: "He respected the first rule of biking."

The bikers stopped midway on their trek across the Prairie Chapel ranch for a 10-minute break near a waterfall on the 650-hectare property, and Bush invited everyone to go swimming in his pool after the ride.

And the President, who also gave his guests riding socks with the Presidential seal, offered understated praise for Armstrong's skills, saying: "He's a good rider."

Armstrong had said last week that Bush rides his bicycle "fanatically" ever since a knee injury forced him to give up jogging.

"I can tell you he is one competitive dude," the cycling hero and cancer survivor told ABC television, noting that "there's no talking" when you ride with the President.

Duffy declined to say whether the President and Armstrong had discussed the bike champion's efforts to promote cancer research, or his coy suggestions that he may be considering a political career and even following in Bush's footsteps as governor of Texas.

Armstrong told ABC that his public ruminations about seeking public office were "more or less a joke," because "it would mimic exactly what I've done, a ton of stress and a ton of time away from my kids."