06 Envoy seating for 3months?????

I've had my Envoy parked in the garage for the past 3.5 months now and want to make sure if I need to do anything before I start to drive it now. Since then I've started the vehicle about once every two weeks and noticed water coming out of tail pipe and under the truck, I would let it run until hot so the water would release from the system, I was doing this to make sure no rusting effects the system. But what I forgot about was the half of a tank of gas seating in the tank, Wondering if I should now use a additive or just run the gas out, then use a tank of higher octane. Is there anything else I should worry about before I take it out and run the heck out of it this week. The Envoy was parked because I just put to many miles on it, I've got 35,725 miles on it.
Thanks for the help guys

Filler up and let it fly. Forget the hi test this will have no effect except on your wallet. Fresh oil is more of a concern.