Throughout my travels in the desert on foot the one overriding factor in determining how long I can stay out is water. It is heavy, bulky and generally one of the first things you will run out of. Here in Arizona where temperatures can easily hit 110 plus during the summer running out can be a death sentence.
Water weighs in at about 8.35 LBS per gallon at that rate how much do you really think you can carry? Take 3 gallons for instance that equates to just over 25LBS, you could carry it but add that in with any other supplies and you are going to be overweight and exhausted in no time.
I Know that for me I can easily burn through a gallon a day traipsing around the desert. When I go out I usually only stay out for a day or two but I always bring extra water and leave it with the truck I go in. Most people think that they can stop at the last 7/11 store on the way out of town and pick up a bottle of water and everything will be just fine.
Well what happens when your vehicle decides to give up the ghost on you and you are stuck now in a hostile environment instead of your nice little day trip. Usually your phone can get you out of trouble but I remember living in Indianapolis and you can be downtown and not have signal so have a backup. One other option you might want to consider is the spot satellite messenger.
Product Description"The SPOT Satellite Messenger, the world's first satellite messenger, uses both the GPS satellite network to determine a customer's location and the SPOT network to transmit that information to friends, family or an emergency service center. SPOT is a revolutionary product that is designed to raise the safety factor for hundreds of millions of people around the world". Please see the add for complete details and options.
I would consider one of these devices for any serious hiker especially if they go out alone. Good common sense like telling someone you know where you are going and what time you will be back doesn't hurt either.
Keep in mind that the above applies to normal non SHTF times. Come a major disaster or adverse social event your whole plan will have to change, different locations mean different styles of preparation but for desert locations water is king.