But at 48 years old, Walker is still looking to athletically challenge himself, and his latest thought is a doozy: he's considering an NFL comeback.
"I know I can play," Walker said on a Monday conference call that was intended to promote his Jan. 29 Strikeforce MMA match. "If I continue to stay in the shape I'm in now, I know I can play when I'm 50. Right now, if you asked if I can play today, there's absolutely no doubt in my mind I can play football today, that I can help out a team today.
"I 100 percent guarantee you I can help a football team out," Walker continued.
Walker, who is 1-0 in mixed martial arts and will face Scott Carson in San Jose, Calif., said his MMA training has him in better shape than when he was in his 20s, when he was wowing crowds in the USFL and later, the NFL.
He said that as recently February 2010, he ran a 40-yard dash in 4.38 or 4.39.
"I haven't been doing my track work, but I think if I get back into my track work and the way I run, I can still run pretty well," he said.
The oldest man to play in the NFL was quarterback/kicker George Blanda, who was 48 during the 1975 season. Only two running backs -- Hall of Famers Jim Thorpe and Ken Strong -- have played in the NFL after their 40th birthday.
Walker is a two-time former Pro Bowler, who played for Dallas, Minnesota, Philadelphia and the New York Giants in a 12-year NFL career. He retired after the 1997 season with 18,168 all-purpose yards, still good for eighth all time.
The Georgia-born Walker said if he came back, he'd like to do it in Atlanta, to have the chance to play at home.
"I still say I may try out for football again and show people I can do that, because I want to be the George Foreman of football and come back and do it one more time," he said.
As for the current NFL season, Walker said he expects the Pittsburgh Steelers to prevail over the Green Bay Packers in the Super Bowl.
"Both teams are playing great," he said. "Aaron Rodgers is playing absolutely incredible. But Pittsburgh has that great defense where they're going to try to shut down the run and force Aaron to throw it, then put the pressure on him. They've always done that.
"Ben [Roethlisberger] is playing really well, too. I've been surprised at how well he's moving in the pocket. He does so well getting loose and throwing the ball. I think Green Bay's going to try to blitz and keep him in there, but I'm giving the edge to Pittsburgh."