A reader sent me this. I don't know anything else about it but I do know this is how I would do it. When we were in Italy last summer one of the things I noticed was many of the hills had old fortified manor houses, not quite castles usually, but not villas either. Why? Because they survived the collapse of an empire that way.
Retreating to a farm is one thing but it has to be in the context of a community with a team of LE/Rangers (Texas type)/light cavalry to run constant patrols and act as a fast reaction team. The place to stop people like this is before they enter your Zone, community territory, or village proper.
The problem then becomes how do you pay them? They will have to be supported by the community through taxation. You can call it tithe, tribute, or a donation but in the end it is a tax. One you will be happy to pay at first.