Former Heisman Trophy winner Herschel Walker last played football in 1997. He’s now 47, an age when many ex-NFL players already are hobbled by arthritis and other degenerative ailments.

But not Walker. Despite what he cheerfully describes as “my advanced years,” he has come to San Jose to train for a new career in the burgeoning brutal sport of mixed martial arts. He’s scheduled to climb inside a steel cage for his debut Jan. 30 in Miami.

And, yes, he knows everyone has a simple question: What the heck are you thinking?

“I wouldn’t have gotten into this if I didn’t know I could do it,” said Walker, a fifth-degree black belt in tae kwon do. “I can fight.”

Walker added that he doesn’t need more money or publicity. But he does need competition.

That’s why he now is spending his days sweating with other MMA fighters at San Jose’s American Kickboxing Academy, which has become a hub for the emerging sport. One grueling session this week saw him repeatedly punching and kicking a heavy bag, then grappling with rising heavyweight star Cain Velasquez, who is 20 years his junior.

Although there are hints of his age in his facial features, Walker looks just as athletic as when he was a workhorse running back who retired as the NFL’s No. 2 combined-yardage leader — right down to a sculpted physique featuring washboard abs.

“I understand why people would hear that he wants to fight and say, ‘Yeah, right,’ ” said American Kickboxing Academy head trainer Javier Mendez. “But he’s not a 47-year-old man. He’s got the body of a 20-year-old. He’s absolutely ripped. He’s not normal. He’s one of a kind.”

Football legend, Olympian, danseur

Walker was one of the most heralded running backs in football history. He won the 1982 Heisman at Georgia before playing 15 pro seasons. Despite eye-catching statistics like once rushing for 1,514 yards with Dallas, Walker is most remembered for the blockbuster 1989 trade where the Cowboys sent him to Minnesota for five players and six draft picks.

He also could evade conventional thinking as deftly as would-be tacklers, always marching to the beat of his own drummer.

A world-class sprinter, he competed in the two-man bobsled at the 1992 Winter Olympics. He danced ballet. He now owns a food company and earlier this year appeared on “The Celebrity Apprentice” reality TV show.

In 2008, Walker also created a stir with the memoir “Breaking Free,” which asserted he had suffered for years with dissociative identity disorder, a controversial mental illness also known as multiple personality disorder. He described how the condition nearly drove him to suicide, destroyed his marriage and is the reason he doesn’t remember winning the Heisman. But treatment brought the disorder under control, he said.

“When the book first came out, everybody would look at me and make a cross with their fingers like I was a vampire because of the stigma,” said Walker, a Dallas resident. “Now when I walk in an airport, I have five people come up to me and start telling their story like I’m Dr. Walker.”

He even uses the condition to poke fun at himself as he talks about his MMA goal.

“This will be my 20-year-old personality fighting in the cage,” Walker joked. “The 40-year-old one won’t come back out until afterward.”

One reason Walker wants to fight is simply because he can.

Always a fitness fanatic, he has stayed in supreme condition with a daily workout regimen highlighted by 3,500 sit-ups and from 750 to 1,500 push-ups. He eats one meal a day — mostly salads and soups, and never red meat — and sleeps only three to four hours a night.

“I could still play football today,” said Walker, who is 6-foot-1 and 217 pounds. “Now I couldn’t take every snap. I’ve slowed down a little bit, but I’m still faster than 80 percent of the guys in the league. That’s why I know I can step into the cage.”

‘Green’ but driven, Walker ‘no joke’

MMA combines elements of martial arts, boxing and wrestling. And San Jose has become something of a mecca for the sport as it gains mainstream acceptance. The promotion company Strikeforce, which is putting on Walker’s fight, is based here. He decided to temporarily move to San Jose because the American Kickboxing Academy gym is one of the country’s top fight camps.

“He’s no joke,” Mendez said of Walker. “He’s green. But he’s got unbelievable ability and is really, really strong. He also has a willingness to learn. You can see why he was such a great football player. He won’t shy away from anything.”

The September announcement that Walker had signed with Strikeforce raised eyebrows. But one person all but chortled with laughter — Dana White, the outspoken leader of Ultimate Fighting Championship, the sport’s top promotion company and a Strikeforce competitor. “Freak show” also is how he dismissively refers to a Walker bout.

“He’s too old for football, but he thinks he’s young enough to fight?” White added. “Fighting is a young man’s sport. You need speed, agility, explosiveness. All that stuff goes away with age.”

Walker is aware of the taunting.

“Dana is just mad because he’s not the only show in town, and that’s fine,” said Walker, who plans to donate his MMA earnings to charity. “But he really doesn’t know what kind of athlete I am.”

The plan is for him to spend the next two months at the American Kickboxing Academy, proving to trainers Mendez and Bob Cook that he’s ready. For now, Walker is a rookie again. New fighters at the gym have to do chores, which is why Walker will be cleaning equipment Sunday morning.

“He doesn’t complain and hasn’t expected any special privileges, which is good because he’s gotten none,” Mendez said.

After a workout, Walker was thanking Velasquez and other fighters for their help.

“I might not be up to the best fighters yet,” Walker said. “But I’ll tell you what: I’m working at it.”

Contact Mark Emmons at 408-920-5745.