The 53-year-old opened up about the setback in a lengthy Instagram post on Tuesday ... saying he's going to do whatever it takes to get back on the board again -- but it's not going to be easy.

"Yesterday sucked," Hawk said. "I broke my elbow 20 years ago and managed to make a full comeback; this recovery for a broken femur will be much harder because of its severity (and my age). But I’m up for the challenge."

We're told the injury happened while Tony was skating ... but unclear what exactly went down.

Tony says there's a bit of irony surrounding the accident, as it happened right before the release of HBO's "Until The Wheels Fall Off" ... a doc that details how and why he's continued to skate into his 50s.

Hawk has previously vowed to keep skating until he literally cannot anymore ... and says this injury will be the biggest threat to his ability to continue the sport he loves.

Despite the serious injury, Hawk's already planning his comeback ... and posted incredibly impressive visual proof of that with a pic and video showing him putting weight on the broken leg just one day later!!

Get well soon, Hawk.