Obama's White House Guest -- 4 MARATHONS AND COUNTING ... After Losing Legs in Afghanistan

There will be an elite athlete in the building Tuesday during President Obama's State of the Union Address -- an army veteran who completed 4 MARATHONS after losing his legs in Afghanistan.

Staff Sergeant Jason Gibson was seriously injured when he stepped on an IED while on patrol in 2012 -- and after 21 surgeries decided he wanted to participate in sports again.

So he learned how to ride a handbike ... and a short time later got himself into good enough shape to complete a marathon.

That was only the beginning ... Gibson says he decided to pick up skiing and surfing as well ... telling TMZ Sports, "I love to go downhill fast. I love speed."

"I started all this after my injury. I tell people, 'I've done more things with no legs, then when I had them.'"

Gibson explained, "Instead of moping around the house, I'm out there being active ... doing something."

"My wife's happy I'm out of the house," he chuckled.

Gibson -- who's been recording a bunch of his activity on his GoPro cam -- says he's already scoping out a few marathons to ride in this year.