And his mountain bike.
Lately, Bush has been logging scores of miles on a secluded spread in Beltsville and the rolling hills of Quantico, Va., far from the White House whirl.
"He's become a biking maniac," said Mark McKinnon, his media adviser and frequent cycling companion.
What began as a way for the president to stay fit, after three decades of running ruined his knees, is now his passion.
Besides burning calories at a 1,000-per-hour clip, cycling gives Bush an emotional rush that sometimes surpasses the one he got from running.
"He's obsessed with it," McKinnon said. "He now likes to do nothing but work out on his bike, and he does it with a frenzy that is reserved for people like Lance Armstrong."
Bush's face lights up at the mention of biking, a heart-thumping release from the stress of his job.
"Prayer and exercise are what keeps me going," he told a Great Falls, Mont., man who asked about his mountain biking.
His penchant for the messy, sometimes dangerous sport captured media attention when he took a face-battering spill from his Trek bicycle - reportedly a $3,100 carbon-frame model - while riding it through the hills of his Crawford, Texas, ranch last year.
"We got thrills, spills - you name it," Bush told an Associated Press reporter who accompanied him on a ride that saw the president sail over his handlebars, crash to the ground and then hop back on his bike.
For Secret Service agents and others who have ridden with Bush, it can be a perilous experience. At least one agent has broken several bones trailing the president.
It can also be demoralizing - he is famous for leaving stragglers behind - but seldom boring.
"It's a rocky, blazing trip. You can't catch your breath - you're not just meandering along," said Vance Holmes, a cinematographer who has shot Bush campaign commercials and videos - and found himself listening to some presidential ribbing after he fell behind during a ride.
Other presidents have also slipped workouts into demanding schedules.
Harry S. Truman favored brisk early-morning walks. Herbert Hoover tossed a 10-pound medicine ball with Cabinet members. Jimmy Carter and most of his successors were avid joggers - as was Bush, until doctors advised him last year to take pity on his joints and quit.
But perhaps no president since Theodore Roosevelt, who combined harrowing horseback rides in Rock Creek Park with subzero skinny-dips, has relished so risky a hobby.
That may be why Bush likes it.
"There's an inherent danger in the mountain bike every time you get on it. It's one of those things where you have to concentrate on doing that at the moment - you can't be thinking about Middle East peace or the future of Social Security. In that sense, it's probably relaxing," said Lewis L. Gould, a retired University of Texas historian.
That appears to be the case for Bush, who isn't chatty during bike rides, save for what McKinnon calls "hoo-hahing" and "testosterone sort of towel-snapping remarks."
"He loves to do this because it gives him mental focus," McKinnon said, adding that "the more stressful things are ... the more he works out."
That may not always be a good thing. An overly zealous bike excursion at his ranch may have hurt Bush's debate performance against Sen. John Kerry, earning him poor reviews and late-night TV ridicule at a critical stage of the presidential race.
Bush "thinks he worked out too hard" the day before the first debate, according to McKinnon. "We really blew it out."
Mountain-biking is an integral part of Bush's fitness regime, which includes riding a racing bike on rollers at the White House and working out on an elliptical trainer and treadmill.
The combination puts his fitness level in the top 5 percent of men in his age group, said Dr. Kenneth H. Cooper of Dallas, one of Bush's physicians and a prominent fitness advocate.
"If a person starts having musculoskeletal problems and has to stop running, they can transition to another type of activity and get the same cardiovascular benefits. I put cycling way up there. It's strongly recommended," said Cooper, who pioneered the aerobics movement in the 1960s.
Washington, D.C., offers few options for a president on the prowl for a fast, tough ride and fewer for agents intent on maintaining a tight security cocoon around his roving workouts.
So Bush has been making weekend trips to the Marine base at Quantico or to Beltsville, where a secure 493-acre Secret Service training facility is perfect for a top-secret ride.
"It's an ideal location from our perspective because there's both privacy and security," said Tom Mazur, a Secret Service spokesman, who would give only a vague description of the James J. Rowley Training Center, to protect security.
The sprawling campus has about six miles of paved roadway, training ranges - including one where the Secret Service mountain bike patrol drills - and plenty of space for off-road adventures.
While the president may be alone with his thoughts on fast-paced rides, he's hardly by himself. An entourage of medics - sometimes including White House physician Richard Tubb - and Secret Service agents with firearms strapped under their biking shirts clatters along. Other agents can be spotted along the route, say people familiar with the rides.
And a helicopter often buzzes overhead, keeping the airspace clear and Bush in sight - even if he gleefully leaves his riding companions in the dust.