Players are often unmerciful to their units, moving them nonstop, going into battle after battle, never a thought for how tired the poor units might be. Although Xconq does not include fatigue as a basic concept, it does have several ways to implement the effects of fatigue.
One way is to use acp debt. If you allow the acp to go negative during a turn, then the player can work the unit really hard for one turn, then it has to rest until its acp builds up to positive levels again. While acp is negative, the unit can take no action on its own. Over a period of time, the effect is that of a unit that can only do so much, but can exert itself when needed.
Another way to do fatigue is via a material type, perhaps called "energy" or "enthusiasm". As an abstract sort of material, don't let energy be passed around (unless you want to have "infectious enthusiasm", might be useful sometimes for leaders and morale builders). Units need energy in order to move, and can consume energy faster than they produce. For instance, if a unit has a speed of 3 hexes/turn, consumes 2 units of energy per move, and only produces 4 units of energy each turn, then on the average the unit will only be able to move 2 hexes in each turn, although if it saves up energy, then it can move the full 3 hexes. Since different kinds of terrain can have differing productivity, you can also make some kinds of terrain be more tiring than others. A resort hotel unit could also be allowed to transfer energy to its residents, restoring them faster than a Motel 6.