Armies are a big part of a lot of fantasy and a good bit of science fiction, but I think that portraying them correctly means considering more than just our own current viewpoint on the topic. In this day and age, we generally think of armies as something that exist all the time and are ready to go at a moment's notice. And that's because we've worked long and hard to make them that way. We pay people to be soldiers and nothing else, and some of the innovations we've made over the years (canning food, for instance) came about because of armies and the need to supply them.
Before these innovations, armies were a different and more troublesome matter altogether. In ancient times, sometimes conflicts came to complete halts because soldiers had to get back home and bring in the harvest. The countryside through which an army traveled could well be completely ravaged, just because the army needed to eat and any crops or provisions that were available were snatched up by the soldiers.
And this has a huge impact on your world and your story. If your army doesn't have canning, for instance, how is it feeding itself when it's marching on the villain’s stronghold? Are the people stopping to hunt (time consuming and not always reliable, ask anyone who's ever played Oregon Trails! ;-) or are they preserving food by some other method and bringing it with them? That would mean supply wagons, which are not fast and require animals to pull them.
Also, a standing army doesn't make sense for a lot of cultures because the cost is just too high. Professional soldiers need to be housed, fed and paid. While they're standing around waiting for a fight, they aren't doing anything else for your society. While some areas such as castles, fortified passes, or guard stations may require a standing force, most areas do not unless there is the immediate threat of danger or a conflict already in progress. Of course, sometimes they can be pressed into service as law enforcement, but that means that if the need to fight arises, your police force is reduced or even nonexistent.
What this means is that, when there is a threat, an army must be raised and equipped. Historically, conscription has been used to drum up soldiers under such circumstances. It has also been common practice to employ mercenaries. Both of these come with their own problems. Conscripted troops are not professional soldiers. While many may have some idea of how to fight, they've probably never fought within an army, which has its own sets of tactics and strategies. If people are conscripted often in your culture, they'll probably be more used to fighting within an army, but at least some of them may not be happy about it being taken away from their homes and families so often.
Mercenaries have their own drawbacks. They're not necessarily loyal to the cause or nation or whatever they're fighting for, so much as the money they're getting paid. They may switch sides if a better offer comes along. Having fought on different sides--or even on the same side--before, groups of mercenaries may be more or less likely to work with one another, depending on how that turned out. Mercenaries may even pull stunts like refusing to fight unless their fees are raised. While it's also entirely possible that mercenaries have some sort of code of honor and consider their work a sacred trust, etc. (or just don’t think they’ll get hired again if they pull that kind of stunt) people being people, there are still going to be some bad apples out there.
So, when thinking about the armies in your world, it always pays to consider a few things: Who's doing the fighting? Mercenaries? Professional soldiers? Conscripted citizens? How do they feel about the fighting, and why are they doing it? How are they paid? How are they fed? How are they housed? Do they have to travel and how is this accomplished?
Don't forget the navy either. It can be overlooked, but if your nations have access to the sea, or even large rivers, lakes, etc., naval attack is always an option. And that brings pirates, privateers and press ganged sailors into the equation. Although, that's probably a separate post!
Problems like these are inherent when it comes to armies, and there are few ways to solve all of them. In fact, instead of trying to solve them all, it can be better to recognize them without fixing them. A culture which loses its policing force when war breaks out can be a valid problem for a culture that doesn't go to war very often. By including it and creating consequences for it, rather than trying to work out why it doesn't matter, you create a realistic culture, and a conflict within it.
In such a culture, some people will then resist going to war for as long as possible, in order to prevent the problem. Some people will want to drum up community spirit in the hopes of keeping crime to a minimum while the army is engaged. Then, of course, you get the people who will take advantage of the situation, who will commit crimes because they know they're unlikely to get caught, or volunteer police forces with their own agendas. All of this creates depth within your culture and cutting out these problems can make your world feel artificial. You can’t, and shouldn’t, solve all of your people’s problems. Instead use them to add complexity to your world.
Soon, we’ll talk about equipping an army, which is a different, but related problem!