Finish the old fuel first, or wait till you can top the tank off
And then start with a new batch at 60z per gallon (it is ok to mix 1/4 to 1/2 a tank) if anything it will run better. Considering the fact that you’ve already ran the engine on this fuel with no issues?
You can always mix new fuel in the tank with new fuel from your jug
But I highly recommend that you mix your 5 gallons of new fuel first
If lack of containers is an issue, then keep the old fuel in a lawn equipment container, that way your 5 gallon jug is empty and free from old fuel mixture
You could always use the rest for your weed eater or blower.
If I understood your question correctly, then this would be my answer.