Some mods help you with that, Diplomacy is a good example as it let you to send recruiters to villages and hire patrols who in turn once they spawn you can command them to reinforce a garrison (yours or other's).
In Vanilla you have to do in the old way, keep your army in the city and then go in a travel village after village. You can train your peasant rabble faster with a group of companions where each had a fair level of trainer skill. Have it yourself if you can spare the points.
Is usually better to have a great number of tier 2-3 troops in the garrison than a fewer number of top units, as the AI take more the numbers than quality deciding what fief attack.