Media makes it look like to le that on one hand there were knights training since childhood while on the other hand foot soldiers were little more than peasants recruited on the go and thrown into the fight as cannon fodder. Although I'm aware that longbowmen actually had to train for a long time to fullfill this position, so a subsidiary question would be : is that an exception/ how would you be picked to do this job as opposed to any other ?
There was a trend toward the renaissance to begin to train soldiers exclusively for war. Also during the 14 hundreds and 15 hundreds, there became a permanently established military force in Italy as well as Hungary under the government of Corvinus, who used this standing army to successfully fight the Ottomans. This standing army preformed very well in battles, which leads me to conclude that not only were they trained well, but became veteran soldiers with time.
The British also began establishing defenses during the viking years to deal with these attacks. As a consequence soldiers were retained to protect certain areas of the British defense fortifications. Source 2 deals with that a little bit.
I have to go, but if you need clarification I can try to do that later.
Interesting Reads:
For English archers, training and ownership of a bow was actually mandated by law. Men under a certain income level were required to own bows and arrows (and possibly swords and armor as well, if they were wealthy enough. The purpose of these laws was to maintain enough men trained in the use of the bow in the general population to easily levy large numbers of soldiers when the need arose. Some of these archers only had their bow, a dagger, and maybe a padded jacket, but the most wealthy of them would be armed and armored for close combat with a buckler, sword, and armor as well. Their status as archers was determined by income. Some of the most wealthy archers might even switch from commanding companies of archers in a large-scale war to serving as non-noble cavalrymen in smaller expeditions (like a border skirmish with Scotland).
Generally, non-noble medieval troops were not just cannon fodder. In the case of mercenaries like the Genoese crossbowmen (and most crossbowmen in general), they were in fact extremely expensive and valuable soldiers, and you would not throw away their lives casually. Hired siege engineers were highly valued for their technical skills and generally well-paid. Swiss pikemen were incredibly good at their jobs and paid as such.
There was no point in taking along men who couldn't contribute to success on the battlefield. Armies (both in the middle ages and today) were expensive and difficult to manage. This was an era before mass mobilization, though, and battles were generally much smaller in size. Raids, ambushes, and sieges were as important as set-piece battles (and more frequent to boot) and did not necessarily require massive numbers of men. What purpose would be served by taking men who can't fight and aren't equipped for war into combat? Every man snatched from the fields is a man who isn't producing food for your population while the army is away. While militias and mercenaries might not have been as well trained as a knight who had been fencing and riding since childhood, they were not unprepared for combat.
Here are some previous questions on /r/AskHistorians that might answer your questions (and address your premise that "foot soldiers were little more than peasants recruited on the go":
Swedish peasant militia were peasants that by law were required to keep and train with arms. Excatly what kind of arms differed by region.
The Södermanna law, compiled into one document by the lawman (an elected position at the thing) Lars Ulfsson (Ama) requires all free men to be able to stand with;
Bow or crossbow with 3 dozen arrows or bolts.
Sword.
Shield.
Spear.
Coif.
Iron hat (helmet).
Chainmail or cuirass.
Villages and thing areas seem to have met once or twice per year to train and practice together.
