I really like your GHB. Well done. My personal preference would be to remove some of material you are carrying to reduce weight. Overall, my thinking would be to get home quicker (by carrying less weight) than to travel those 20 miles prepared for just about anything.

I see that above you said some of this wouldn't be making a trip with you but I can't help myself and so I'll make some suggestions. If this were my kit designed (only) for me to walk 20 miles, I would drop many things to save weight (and possibly carry more water).
For example:
- 2nd pair of EMT shears
- Playing and plant cards, Ref Guide, Bible (I am no doubt cursed for this thought)
- many of the food items like the jerky which would give me heartburn and make me carry extra Tums (have you considered a blister pack of Prilosec or similar medication instead?).
- extra lights and mostly reduce the spare batteries.
- most of the quarters (use a credit card to make a phone call or call collect)
- sharpening rods and the base (maybe keep one rod?)
- the small pry bars and Nite-ize 9s

Consider adding something like Moleskin or other blister products. I see you have extra glasses. Are they sun glasses? One other thought would be several feet of clear tubing to use as a siphon or that might allow you to suck up canal or lake water without risking a tumble down a steep concrete embankment.

The most bizarre thing I could suggest would be some sort of sun umbrella or other shade that you could rig to your pack to give your entire body shade. This could make a huge difference in Phoenix. As an added bonus, people might think you're nuts and stay away.

Finally, perhaps some goggles for any sandstorm you may have to walk through.