The Kansas City Chiefs legend was diagnosed with a learning disability at a young age ... and used track and field at Special Olympics events to help him get through tough times.

In fact, the running back -- who's the all-time NFL leader in yards per carry -- says, "Special Olympics is one of the best things that ever happened to me."

Charles says the events helped him deal with bullying and other issues that came with his disability, and tells us he couldn't be happier the organization's government funding remains intact.

"When you go to some of these games right here, these kids will just put a smile on your face. Winning is really nothing to them. It's just really just having fun."

Charles' message to all the Special Olympians out there? "Don't give up. Believe in yourself."