The Fyrd were not full time fighters.  They were called up when the King needed them.  They were not peasants but trained men,  chosen because they could be useful in war.

All free men in Anglo-Saxon England had an obligation Definition to serve in the Fyrd army.  Important men who owed the King allegiance Definition would provide the King with men to fight in the Fyrd.

The Fyrd
from the Bayeux Tapestry
The moustache shows they were Saxon!

In 1066, as King Harold marched north to internal link Stamford Bridge, men in the towns he passed would have joined the Fyrd.  As King Harold internal link marched south, these men would have gone home, and others joined as Harold got further south.