Large armies have always travelled slowly compared to small groups, even the comparably mobile Mongols. Just think about the amount of time it takes for a large group of people to get up, eat, organise, file out of camp at the start of the day, forage, set up camp, file into camp, etc etc.. While travelling they'll be limited to the speed of the slowest units. They'd be limited in the route they could take to areas where the army could feed itself. And that's without doing all the stuff like fighting, negotiating, pillaging etc. that armies are meant to do.

Individuals and small groups could travel over a wider variety of terrain (eg. narrow passes) or take boats more easily.

In terms of how information would spread, it would be a variety of means. Traders would probably the biggest travellers and main conduits for rumours, but you'd also have contact between officials and rulers in different regions, particularly in comparatively well-organised China.

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