As Su Rong had expected, Han Yin relaxed his guard after he escaped and saw no troops sent by Su Rong chasing him.
It was not his fault for relaxing his guard. From Su Rong's perspective, the Ye brothers were seriously injured and needed to recuperate, the captured troops needed to be reorganized and there were plenty of post-war matters to handle. Besides, she had her own troops to deal with the Nangong Family, so, as a defeated soldier, he was probably not considered a big threat by Su Rong.
He accurately assessed Su Rong's mindset, but he didn't know that Su Rong had Qin Ruo by her side, who reminded her where she had overlooked.
So, he didn't march his forty thousand soldiers day and night. Although they were moving at a reasonable speed, they weren't particularly fast.
Unknown to him, Su Rong had already set out with one hundred thousand soldiers, taking a shortcut to intercept him.