Some of this is not a training problem, and treating it like one wastes time your dog may not have. If they are distressed when you leave, if they growl or guard their food, their toys or you, if they are frightened on the lead, or if anything about them changes suddenly at any age, contact your vet the same day. A tip from a website, ours included, is not the answer. If it cannot wait, contact your nearest emergency vet now.
With a newly adopted desert dog, fear and food guarding in the first weeks are common. If anyone has been snapped at, or if either is getting worse, contact your vet. Once they have settled, watch for change. Going quiet, stiffness, snapping when touched in one place or losing interest in their walk can be the first sign of something physical. Bloat (GDV, a twisted stomach) can happen in any dog, though it is most common in large, deep-chested breeds. A swollen, tight belly, retching with nothing coming up, sudden restlessness or drooling is an emergency: contact your vet now.
They'll always know walk time before you do. If they stop asking for it, tell your vet.