If Zhang Yanhang had rashly chosen to enter the Ashen World immediately, he would have faced a very severe situation. He would neither be immune to the zombies' attacks nor to the T-virus. As a result, Zhang Yanhang would most likely have tragically died in the Ashen World and even been transformed into a zombie after his death.
Moreover, if he had come half a month earlier, he wouldn't have encountered bizarre monsters like the Goblin Merchant who inexplicably ventured into other monsters' territories.
"It wasn't in vain that I waited half a month before entering the Ashen World. Being cautious indeed has its benefits."
At that time, Zhang Yanhang didn't know that he would come across something like the Earth Spirit Fruit. His main reason for waiting was because his Limitless Universe Skill was about to break through. So he thought of waiting a bit, consolidating his progress after the breakthrough before heading to the Ashen World.