Yeah, the crux of the Fabian strategy is that time is on your side. The further the coalition armies get led into Qin, the further their supply chain has to be stretched; It is impossible for them to maintain their army unless they receive assistance from nearby towns. That is why XSL would evacuate the towns in the way. Other armies in history would burn and salt the land, thus the term scorch earth. The weather is also worsening, and the coalition army is probably not prepared for a winter campaign, while the Qins are used to the condition since this is their homeland.
Plus as long as you don't engage the coalition army directly, they can't attain any big victories, and their morale will be affected. They are only working together against Qin for their own benefits, there is no loyalty within the combined armies.
Also of note, I am surprised their wasn't some emotional good bye scene for XSL with his wives before he went off to war. It seems that's all his wives are good for now that he's won them anyways.