An extended warrenty is seldom worth the paper it's written on. There are usually many restrictions and hoops to jump though in the event it is needed. Miatas' in particular are very reliable and should run for many happy miles with minimal attention (that is, oil changes as recommended, brake flush every couple of years, coolent change as recommended. Don't waste your money.
As far as milage, remember it is a sports car, not a Prius. I have never gotten more than high 20's in any of my Miatas.
Remember to enjoy the drive, they are wonderful little cars.