I accidentally hit "X" (Mark Clip) and I highlighted a clip. How do I undo it?

I don't really care (????) but you're supposed to mark the post "correct answer"  that solved your problem not your own post that's saying "thank you."   But glad you've solved (or should I say "we") solved your problem.   Kind of sad that it matters...  but I'll get over it.

don't usually get the opportunity to correct Ann, but I think this depends on which keyboard preset you've chosen.  Actually just did a search for in the keyboard shortucts dialog for "undo selection" and nothing comes up.  

and you can change your export options to export your entire timeline.   When you're in the export dialog on the bottom center left side, simply change the option from source range from sequence in to out, to entire sequence. 

Not sure how you're keyboard is mapped but you want to clear in and out.  If you go to the premiere menu and choose keyboard shortcuts and do a search for "clear in and out" if it's not already mapped to a keyboard shortcut, you can map it to wherever you want on the keyboard.  Both mark clip and clear in and out are very useful keyboard shortcuts...