There is no rule of thumb on how many miles you will get out of a tire. Ask the LBS to show you how to spot signs of wear, as others have said worn tires will flat more. You are putting air in the tire every ride, right? If not you may run the risk of pinch flats. I had good luck with the Conti Gator Skins, not sure I ever flatted on them. The Specialized Armadillos are also a good tire. I figure weight won't slow me down nearly as much as a flat. I am not a weight weenie, the engine (me) of my bike is where I need to shave weight!
I do road ride alone, before DH would let me he taught me how to change flats. Sounds like you know how to do that and I can't blame you for being tired of doing it! Can you take a basic maintenance class? Several of our local shops and REI offer them. I find knowledge is power, knowing if something were to happen I can take care of it makes me less nervous. I worry about wrecking but then I remember how many rides I have done and how few incidents I have had. To be comfortable alone I do let someone know where I am going and how long I expect to be gone. If it gets too far past my time they usually check on me.
Amanda
2011 Specialized Epic Comp 29er | Specialized Phenom | "Marie Laveau"
2007 Cannondale Synapse Carbon Road | Selle Italia Lady Gel Flow | "Miranda"
You don't have to be great to get started, but you do have to get started to be great. -Lee J. Colan