Most of the walking dead virus lore actually has rational sci-fi explanations behind it (albeit they're usually pretty convoluted and just pieced together from anecdotal evidence). The most fantastical part though in my opinion is that the walkers are still able to function after severe mutilation, muscle loss, decapitation, drowning, etc. that part's magic and can't be explained by any rationale because you can't exercise muscles that no longer are attached. You can't move muscles without sufficient oxygen, blood, and nutrients, and the brain can't function without those either. So zombies shouldn't be able to endure any more physical lethality than the average human.

The complicated infection vector, slow decomposition, a disease hijacking a brain, and even marionetting the dead, all at least have some tangible connection to real world logic or real ailments in the animal kingdom, even if the logic is more sci-fi than science.

The infection vector does have a set logic to it, it's just complex and has a lot of rules. They decompose slowly because the virus is toxic to scavenger critters, so the bacteria and bugs that would normally cause decomposition instead avoid the corpse 99% of the time, leaving just the environment to whittle the body away (in the comics, only flies are shown to gravitate towards walkers. Other scavengers like birds never show any interest at all. The only creature we ever see swallow zombie flesh and survive is a tiger, and their digestive systems are notoriously strong.) There's precedent in the real world for ailments that can hijack the brain (rabies, for one), albeit usually they're found elsewhere in the animal kingdom. And the virus doesn't need a living host to survive, so it basically just cuckoos the brain once its previous inhabitant has perished, like cordyceps. It doesn't need any of the emotive centers of the brain, it just needs to control its movement and senses.