Boiling excess water?

I am brewing a 5 gal batch w 2.5 I'm the pot for the wort. I've boiled the other 2.5 gallons of water to mix and left it on the stove to cool for 2 days w the lid on. Ok to do?? Will there be bacteria risk since it took a lo.g time to cool down and /or has been sitting out (w lid) for 2 days??

I myself would prefer a shorter window between boiling and use time, but that's just me. More than likely, it's fine. Especially if the lid of the pot has any kind of a gasket on it like mine does (and the lid was placed over the boil pot to sterilize).

You could also use some bottled water from the supermarket (reverse osmosis or distilled) for your top off water, or you could add a campden tablet to your water to help the yeast take hold and dominate the beer. Remember, your goal is to sanitize so the yeast can take over enough to produce alcohol, not necessarily sterilize all the life from your brew setup.

Are you saying you just boiled the water and left for 2 days? If this is the case, just drop a campden tablet in to help assist or maybe some meta to act as an anti-bacteria preventor.

FYI - you are going to need more than 2.5 gallons to top off. The 2.5 gallons in the pot will boil and evaporate for an hour. It is always a good idea to have some extra clean water on hand to make up for that.