Once your nation gets big enough, you can't supply frontier armies from the capital. You need to draw food from local sources. In territory you control, the peasants pay some of their food to the local fortress in taxes, and the fortress acts as a supply base for any army that needs to go through.
Once you get past border action and deep into enemy territory, the tyranny of the wagon equation means it's down to "forage" -- hunt / fish / gather resources from the wild, or steal food from the local population at swordpoint.
One option is to pause your campaign until you can consolidate your gains and build or take over supply bases in captured territory.
Another option is to change your tactics to recognize the limits of what a foraging army can achieve. E.g. avoiding sieges and terrorizing civilians / burning towns to try to force a surrender, or using a cavalry army or fast-moving elite infantry to launch a surprise attack in an unexpected location.
Once you get past border action and deep into enemy territory, the tyranny of the wagon equation means it's down to "forage" -- hunt / fish / gather resources from the wild, or steal food from the local population at swordpoint.
One option is to pause your campaign until you can consolidate your gains and build or take over supply bases in captured territory.
Another option is to change your tactics to recognize the limits of what a foraging army can achieve. E.g. avoiding sieges and terrorizing civilians / burning towns to try to force a surrender, or using a cavalry army or fast-moving elite infantry to launch a surprise attack in an unexpected location.