I think people exaggerate how rare pitched battles were. In most major wars in medieval Europe, you see at last one pitched battle. That said pitched battle alone did not necessary decide the war as pitched battle was not war goal on it's own. Given most of the time war goal was control over territory and resources and territory and resources were usually protected by some kind of fortifications, you needed siege to reach your goal. So pitched battles were important as far as they either led to a successful sieges or prevented enemy from successful sieges. There are number of examples of military campaigns and pitched battles that did not result in successful sieges and control over territory/resources. Notorious example are English campaigns in France during Hundred's Years War. English often won major battles, sometimes in spectacular way, but they sometimes failed to transform those victories in to territorial gains and control.

So how would major military campaign of the Medieval era look like in Europe? Feudal lord, for example king would declare war over some (usually territorial claim). He would set up a war goal, usually town or fortification commanding that territory. He would call up to arms his subjects and set a place where troops would gather. This might take few months. Preparations would have been done in advance (could be as much as year or half a year in advance) to gather equipment and supplies (may include ships). Once assembled, leadership would be assigned and march routes planed. Then army would march to it's objective. On the way army would conduct raids and siege any fortifications and settlements that would pose obstacle towards objective and to secure lines of communication and supplies. Once at the objective, siege would begun.

Opposing side in the mean time would start assembling it's own forces, typically with a time lag of few months. If force assembled was deemed large enough to confront the attacker, army would march at the enemy with the goal of either preventing him from reaching his goal or lifting his siege of the objective if he already reached it. If nothing else intervened, armies would meet and fight pitched battle. If attacker lost, war would be over. If attacker won and his force was still strong enough to carry on to the objective, he would proceed with his goal. There was still the chance that he would fail his objective however as he might still have to fight the siege. There was also chance that even after victorious battle, losses would be such, that attacker would not have enough strength to capture the objective, in which case he would fail.

If assembled defending force was not deemed large enough to confront attacker in open battle, it could still try to frustrate attacker's siege of his objective. They could try to de-blockade siege town or fortress, reinforce it, raid enemy rear, set up ambushes, prevent him from foraging effectively or just simply pose threat by been present nearby.

In some instances defender might fail to assemble his force or assemble it on time and attacker might have already taken his goal. If defending side refused to accept such outcome, they may try to mount their own campaign later to recapture lost territory. Sometimes one or more years later.

Thanks to character of feudal mobilization and economy, feudal warfare was extremely indecisive. Meaning it was very hard to defeat enemy decisively on a strategic scale. Forces that could be mobilized at any moment in time were as a rule only fraction of actual military capacity of the country. Large part of feudal levy was immobile and incapable of leaving far from their homes because of the lack or transport and seasonal character of agriculture. And even more mobile part of levy composed of feudal lords, their retinues newer assembled in full when called. Therefore feudal armies were small in size relative to overall military capacity of the country and were restricted in time they could effectively spend on the campaign. Therefore even if such army was defeated in the field, it was always possible to assemble a new one over time. Add to that fact that movement and communications were slow, feudal wars tended to drag on and resemble pendulum: Side A would assemble force and attack side B. Since side B started to assemble it's force later and with a time lag, side A would have initial advantage pushing forward. However over time force of side A would diminish while side B would have assembled it's own force gaining advantage in turn. Rinse and repeat. Therefore large feudal wars tended to go forward and back for years with sides gaining and loosing momentum as they gathered their war resources, spend them, retreated back to recover, gathered their resources again and so on.

Because of above, lot of medieval warfare was actually raiding. In fact some military campaigns had raiding the territory as a goal, rather then capture of that territory. This was to some part also caused by character of motivation of Medieval soldier, for whom war booty was important part of warfare. Over time such raiding could diminish resources of the opposing side and could be more effective then any pitched battles.