This is about is your ability to carry as much weight as possible for the entire time you will be out in the field. You can take as many breaks as you want, but don't take more than you can physically stand.
Imagine that you are going to have to carry everything you need on your back. Now imagine that you are going to have to keep moving for the entire period of your stay in the area.
MAKE SURE YOU CARRY ENOUGH WATER TO LAST THE DAY!!!!!!!
Before doing any serious desert or mountain hiking, put 40 pounds in a backpack and hike around your neighborhood for about 4-5 hours or 10 miles. If you can't do that comfortably, you have a couple of choices:
Now, put together a kit of everything that you will need to carry with and on you (Medical Kit, Rock Hammer, Knife, Pliers, Hat, Clothes, Sunglasses, Gloves, Snake Leggings, Boots, Socks, flashlight, chalk, camera, etc.). Figure the weight of all that, and then add the weight of your water. It starts getting heavy very quickly!
There is much more, but this will get you started on the right track to working with Mother Nature to help you stay alive.