I am also nearing Kweiyang. My forces have spread out fast once Changsha fell.
One of the secrets of making an offensive this fast is to always have plenty of reserves, and a way to get them to the front fast. I have been using the TO&E for this, but not like one would traditionally.
In the traditional model you would have troops flow from the top HQ (in Tokyo) to the HQs in Shanghai and Peking, and from there spreading out to the Army HQs, and then finally to the troops. Each of these transfers being a step and thus a turn. This is in my opinion too slow.
Instead I chose to send my production straight to the Peking and Shanghai HQs. The HQs there would then send to the armies, and I would send the forces that were sent there straight to the units, with a direct transfer. Thus it takes much shorter time to get there, than it traditionally would.
Sometimes I would even forgo sending them to the HQs in Peking and Shanghai and instead send them directly to the HQs in the field. I did that especially when making new Infantry, Armor or Artillery divisions.