They wouldn't get paid. It is a duty to the King to fight in your own Fyrd, under command of the local shire Reeve. Not showing up was punishable, so you'd better have a good excuse.
I don't think fyrds would travel far beyond their own shire, so they were mostly defensive. I feel like the furtest extent would be 2 weeks max. Just imagine carrying a weapon, a shield, and food for 2 weeks. You just can't begin to image how hard it was to keep a army in the field during the dark and middle ages.
Napoleon said something like: wars ain't won by guns but by logistics. And that was 1800. Just to make a point. No railroads than