Adding gas/oil mixture into unmixed gas tank?
I think this should be fine. The mixed gas will be diluted almost 10x if you fill the rest of the tank with fresh gas. The mower might be a little smokier than usual but you probably won't even notice.

If it makes you feel better you can keep topping up the tank with fresh gas as you use it, that will further dilute the oil. But personally I wouldn't bother.