I’d bring more water, but I do all my hiking in the Canadian Rockies and would wilt in the heat where you’ll be hiking; you may be more used to the heat. The light shelter’s a good idea, for shade if nothing else. Take care of your feet (you’ll be wearing light hiking boots, yes?) and keep the sun off your head, and it sounds like you’ll be fine.
Heat, humidity, sun and no shade. Challenges yes, but you’re preparing well. My suggestion is for a piece of gear, a flax desert water bag. Like these.
Back in my military days we lived and trained in the extreme heat of the Mojave desert. The best-ever water I tasted was out of one of those, because the bag perspires and keeps the water cool. Soak it empty, inside & out, fill it, and hand it in direct sunlight - not in the shade. Make sure the bag’s skin is saturated. It’ll drip and sweat, but that means it’s working (a dry bag means hot water). So my suggestion is to bring an extra gallon of water on the way out. It’s only an extra 8lbs at the beginning, when you’re fresh. Hike out about 3-5 miles, then prep and hang this extra gallon in the bag in a secure, hidden location. It’s best if the bag hangs freely but if you have to lay it on the deck, so be it. Then continue your hike.
Plan on the bag NOT being there on your way back (it sounds like an out-and-back route, if not then this plan is worthless) - ration your water such that someone else may find and take the bag. But if the bag is there on your way back, and if the heat and humidity have been bad, then this cool water will give you a good charge to finish strong.
One last thing, these bags are like swamp coolers and work best in low humidity. I’ve only used them in dry climates, so cannot vouch for how well they work when it’s Gulf of Mexico humid. But I think it’d work well enough.
But yeah, the best water I ever had was when it was 115°F out, drinking cool water from one of these. Have fun!
All good stuff here! Too cumbersome to quote everything on tapatalk.
I think i have made up my mind to leave in the evening, do the “out”, and sleep before coming back. That’ll give me time to recover a little, plus being 5 miles from anything will be cool. Stars and stuff.
Going to load up the pack and do 10-12 mile test hike on gravel this weekend, that’s the closest approximation I can get.
I procured Bates desert style boots, I wanted the higher tops to keep sand out. Also, they’ll provide ankle support on the side slope and if I need to cut across deep sand.
Think I’ll look in to the water bag, that sounds like it could be nice.
I’ve never used trekking poles, but I figured it can’t hurt so i have a pair on order.
Nobody tried to talk me out of it, so guess I’ll do it. I’ll post from the western tip of the island at midnight in a couple weeks 
One more thing. Do you have one of those portable chargers? Like a Halo?
Procured, you mean borrowed? It’s important you already know they’ll fit well and won’t blister. I’d go with something much lighter weight, perhaps this style, but do what’s right for you.
The flashlight, an LED can be very lightweight and bright.
One last note, some shoreline areas have a lot of rattlesnakes. Keep an eye out when you’re in the back dunes and scrub brush.
This sounds a lot like something I would have done 20 years ago… have fun. Obviously you’ll see things few others have.
I don’t, but my phone battery lasts an insanely long time. I can fiddle with it all day and still have battery left at dinner time. With WiFi and GPS off, it will last 24 hours.
Trekking poles can be great (I have a bum knee right now and use them) but it’s really just extra weight on flat ground. It takes some load off your legs but I wouldn’t bother with them for this type of hike. After a while they’ll just be more to carry and I’d rather keep my arms relaxed.
If you anticipate hiking in the night get two small LED headlamps. You don’t want to be carrying a light in your hands.
If you are carrying two full 80z bladders and a quart of sports drink that is starting to be a pretty heavy pack, and being in sand is going up the difficultly level. Make sure you take A LOT of food. Like probably more calories than seems reasonable. Hiking with a pack that heavy, and in sand, its going to be a lot harder to maintain 30 minute miles for 10 miles than you think (and your going to want to drink water like a fish and eat like a horse when you’re done).
Not saying it’s not doable, just saying don’t underestimate the amount of food you’re going to want to scarf down at the end of the first day to recover and be ready for the return trip.