IMO, the role of the general is to manage all of the armies. In most realms, where there's only 1-2 armies, that means very little and they usually end up stuck doing diplomatic work with other generals instead or, if there's no alliances in place to take advantage of that, basically taking over as a Super Marshal. "Go here." "Do this." etc. - with the marshal just picking a formation to fight in, or assigning line settings as given to him by the general. What the general does in a realm with multiple armies is, IMO - again, organize. He orders armies to one sphere or another (not necessarily specific regions, but target areas) and gives them general orders - to raid, to be aggressive, to seek out the enemy army, to lay ambushes, etc. He determines who refits, and the roles of each army, but leaves the specifics up to each marshal.
The marshal, in turn, must be proactive and aggressive. They must be capable of taking these broad orders and - with a large degree of autonomy - going out there and accomplishing them. If they're told to raid in X duchy, they should plan with their vice-marshal and then go and do that, splitting their own army or not as needed. If they need to join up with another friendly army assigned to their target area to defeat or defend against a larger force, then they should do that immediately without needing to wait for the general's say so.
The general is a staff officer. They assign broad targets, and receive reports on effectiveness and accomplishment from their marshals, replacing and promoting as needed.
