"I have a 12 hour endurance event on Friday" with an electrolyte "imbalance"? This would have me concerned if it was me racing. It's my understanding that heart regulation and potassium are tightly intertwined.

Can you maybe get a stress test prior to the race or at least have a doctor sign-off your your race?

If you want to self doctor yourself you can get a lot more potassium in "no salt" that you can get from those pills. I hate to give you advice like this but you can at least look into it anyway.

You are a smart, competitive lady. I trust you will make a wise decision. I do NOT want to disuade you from racing Robin, these are just thoughts that popped into my mind.