Go back and re-read the scenario.  This whole process takes a while.  There is an interim period where Food Aid is still moving from farm area to urban areas organized by probably the military/national guard.  Not enough food for everyone, so you have the gang warfare period.  As the population dwindles through starvation and gang wars, you get outward migration.

Say the city is Springfield, MO.  Surrounding the city are many farms in a 100 mile radius.  The city dwellers form a gang, they hop on their bicycles and ride out.  They hit a couple of farmhouses, kill the farmer and family, steal the food and/or eat them.

So now the farmers form a self protection militia as a coop venture.   They have patrols watching for gangs coming out of the city.  Some farmers and some gang bangers killed in each attack.  Population dwindles more.

Now farmers need help with planting and harvesting.  Make truce with gang bangers for protection and labor.

This is exactly how it worked in the middle ages after the fall of the empire.  Cities like Rome dwindled in size and there were roving marauders all over the countryside.  So they started building castles and walled cities with farms around them.  When marauders showed up, they ran inside the walls.

If the marauders could mount a long enough siege, they could starve  out the farmers.  Sieges went on for months at times.

Truces were called, allliances formed, armies developed.  But it all takes generations to play out.

RE