Is there a version of "and now if you'll excuse me" that's more informal? I am trying to find alternate way of saying "goodbye", but "and now if you'll excuse me" sounds way too formal and doesn't sound like something that two old friends would say to each other. Is there a more informal idiom?
If you want to let someone know in an informal way that you need to leave, you could say
I’ve got to go!
A couple of expressions we frequently use (USA) are "gotta go", "gotta run", "need to get going", "I better get moving", plus tons of others I can't remember right now, some very picturesque. You could add "if you'll excuse me" to a sentence containing one of those, and it won't sound terribly formal. Also, "I'm sorry but," is a roughly equivalent, but less formal alternative to "if you'll excuse me,"...
"Oh dear, is that clock right? ... Listen, if you'll excuse me, I gotta get my butt in gear and get moving. I'll see you on Saturday and we can talk more then. OK?"