Bush, 67, planned to ride the full 100 kilometers (62 miles) of the fourth annual Warrior 100K, accompanied all the way by the current and retired military personnel who were wounded in Iraq and Afghanistan.
"The thing that amazes me is that rather than allowing life's difficult circumstances to overwhelm them, they've turned the circumstances into good by showing courage and hard work and sacrifice," Bush told reporters before heading off on a 30-mile ride on his ranch in Crawford in Central Texas and along nearby country roads.
Friday's route zigzagged through the property's wildflower-strewn prairie and along a trail on a ridge overlooking a creek.
"I'm surrounded by extraordinary people, people who have volunteered to serve our nation and in so doing became injured," he said.
U.S. Army Master Sgt. Chris Demars, who was wounded in two separate attacks in Afghanistan in the space of just 10 days in 2011, was among the riders. The attacks - a roadside bomb and a motorcycle suicide bomber - left him with traumatic brain injuries, two broken legs and multiple shrapnel wounds. He also suffered a back injury and both his eardrums were blown out.
Demars, 45, has since fought to overcome post-traumatic stress disorder along with his many physical ailments, and says he admires Bush's efforts to reduce the stigma of the condition. Bush asserted earlier this year that "disorder" should be removed from the condition's name, underlining that it is entirely treatable.
After being flown back to the U.S. for treatment, Demars, of Gill, Massachusetts, was recovering in a hospital when he saw news coverage of Bush's first Warrior 100K. He told his parents he would take part the next year. It took him three years to recover sufficiently to join the ride, which he described as an "awesome" experience.
He said bicycling provides the kind of excitement that he misses from overseas assignments.
"You get out there and you hit a good descent with some tricky spots and you almost crash a couple of times - it's good. It keeps that adrenaline rush going," said Demars, who noted that such exercise is also a good way to combat times he's feeling agitated.
The ride is part of the Military Service Initiative organized by the George W. Bush Institute, a part of the George W. Bush Presidential Center on the Southern Methodist University campus in Dallas.