Tony Hawk says a failed attempt at a popular skateboarding move led to his severely broken leg on Monday ... and now, the skate legend says he's hoping to one day be able to walk again "unaided."

The 53-year-old finally went into detail on Thursday about what caused his broken femur earlier this week ... and he says an effort to do a McTwist (a front flip with a 540-degree rotation) in the half pipe busted up the limb.

Hawk explained he had "very little speed and an unsteady grab" while trying the trick -- and when he came crashing down on the ramp, his leg crumpled.

"I hit the wall sooner than expected in a half-squat position and it sent me straight to the flat onto my knees," Hawk said.

"The force of my body position upon impact broke my front leg as I came sliding to a stop."

Graphically, Hawk said he had to "maneuver" his leg back into place -- before an ambulance eventually transported him to a hospital.

As we reported, Hawk underwent surgery on the leg right away -- and was somehow able to put weight on it just a day later.

Tony, though, said Thursday he has a long way to go to fully recover ... explaining, right now, his goal is just to be able to walk normally again.