It took Sylas until the next day to finish. He believed that the reason it was so difficult was for two main reasons.
The first were his Runes. They were small and unimpressive compared to the vastness of the mammoth that he was dealing with. He believed that he could make them larger and carry a greater capacity if he were to use more Aether and more effort, but he felt that the return wasn't worth it, especially since he was technically still in a dangerous situation. It was in his best interest to stay as close to 100% as possible.
The second reason was the reference he used to make the Runes in the first place.
He had learned a lot about the fundamentals of runes, and one of those teachings was about the different categories and grades of them.
Runes weren't as straightforward as Levels and had three main facets to them.
The first was Grade.