Well, you wanted what you wanted, so you cheated. Now you want to know what to do so that you can keep everything you got. You are being selfish. You need to come clean to your husband for betraying him and lying to him, and you will have to work very hard to make amends and regain his trust. There is no way to keep your relationship healthy without honesty.
Regarding K, it's so very convenient that his gf "would never understand," so he just won't tell her. Of course not! He got what he wanted, too. I wonder how many other Craigslist penpals he secretly fucks?
I think you lost your head and got yourself into a bad situation. Whether out of frustration or complacency regarding your marriage, you chose to focus elsewhere instead of working on what needed fixing. Now you have to lie down in that bed you made, as they say. How honorable of a person do you have the courage to be? Time to step up.