Just filled up, and they gave me regular when I asked for premium. Should I run the regular gas down before I refill, or run it to half the tank to diffuse the regular gas with the premium my engine recommends?
It depends on the engine. You would get much better information if you looked on a site or forum specific to your car. I have no experience with the Toyota 2ZZ engine specifically but it being a modern engine with knock feedback, I would assume it would be fine to do as long as you don't drive the motor hard.
Being the 2zz engine the effects will be minor on your engines since its only one fillup... Dont hit lift during this tank and probably shouldnt rev over 3k rpm until you get a full tank of premium in..... You can also buy a bottle of STP octane booster to help your regular gas run a little better.
I also would suggest the octane booster if your owner's manual requires the 91 octane. As BDC points out your onboard computer should have a knock sensor that will retard your ignition timing, alter your fuel mapping, possibly change your valve settings if your engine application has that option any one of which will affect your engines performance but should protect it from potential damage from using a lower than specified octane rating. A bottle of Octane booster from your local auto parts store shouldn't be too costly, pick one up, add the amount recommended for the amount of 87 octane that was added at that fuel up and top it off with 91 to mix the booster into the tank and you should be fine.