Doc Comfort comes closest to the meaning as I understand it. If I bragged, for instance, that I would destroy the local street punk if he tried to push me around but then, when said punk relieved me of my wallet and spit into my face for good measure with me putting up not even the feeblest defense, an on-looker would rightly say "Don't talk the talk if you can't walk the walk." In other words, deeds speak louder than words. Another colorful expression that applies would be "Don't let your mouth write a check that your ass can't cash!"

Unrelated sidenote: The spell-check doesn't recognize the word "punk" and suggests I might mean "puns" instead. I've never been assaulted by street puns but, frankly, don't think I would mind it overly much.