I told my employer and all my work colleages I had a really boring hobby, Chemistry (something I am actually interested in)
Then bang on about it every chance you can get. Go into elaborate detail about the most boring parts. After a few weeks people will learn to leave you alone.
Make sure you always have an answer for "what are you doing tonight", "what are you doing this weekend" be super excited about it, but make sure it's really boring.
Then when it's time to leave tell them you're going travelling around Europe and you have no idea what job you will do next.
Launch your product, and never work the 9-5 again (it's a new type of work now)
That's what I did.
I know people are gonna say "Thats risky", "You could get sued" etc etc etc ... but you know what? Life is risky, and if you dont have the guts, my answer to you is that this is the least of your problems running a startup : maybe you should stay in that 9-5