There seems to be a couple issues you've asked about: How can the virus spread into a full blown apocalypse, and how can virus zombies survive for long durations in the elements.
Spread is the easy problem to solve: give the zombie-ness as the last symptom of the disease....one that doesn't occur until after the person has been infectious for some time (days? weeks? months? you decide. Longer the period the more dangerous strangers are in your world). The virus itself is interested, biologically, in spreading as much as possible. It could have evolved as a "slow burner" that starts off as a relatively mundane infection, perhaps causing higher blood pressure or some other small symptoms.
As the disease progresses, it changes the bloodchemistry, causing heightened "fight or flight" responses to other living things. Simultaneously, absent of these stimuli, the bloodchemistry slows the metabolism. Add in some brain lesions in certain areas, and the person will see every living thing as "food" even if they retain some of their critical thinking skills (much like animals that are infected with parasites retain their abilities to move, but lose inhibitions that keep them from being eaten). Eventually this goes full blown zombie, viciously attacking any "meat" that comes near. The virus spreads from any fluids and tissue of the creature, and could even be airborne if you wanted. The complications of a water or airborne virus would be difficult to handle indeed.
If the body is kept more or less intact, and can be fed (by the zombie viciously attacking any "meat" that comes near), then it should be able to survive for some time. This can be enhanced by lowering the metabolism of the organism for some time, until spiking it with adrenaline in response to stimuli (there are instances of snakes spending many years sealed in wine bottles that wake up and attack the opener of the bottle. Obviously, we aren't reptiles, but bears hiberate, spending months not eating).
In terms of infection, the virus itself could provide safety there. Perhaps this virus enhances the immune system of the host to fight off infections, or attacks bacteria itself. You'd still get the "dirty zombie" look, you'd just not get the "rotten zombie" look. Add in biochemical changes that improve the body's response to trauma (enhanced swelling to reduce blood loss, or smarter clotting to seal wounds relatively quick, or both).
Exposure might cause some issues, but there's no reason the biochemistry changes that affect the fight or flight response and metabolism can't also introduce some form of resistance to frostbite or heat stroke. You're already going to be lowering body temp by "hiberating" when stimuli are absent.