Hello to you too.

The thing is, I've been playing Heroes for a long time, but pretty poorly — I don't understand strategies, combat tactics, magic, and other important things at all. I usually play by trial and error, however it happens — I have no specific game plan.

As I understand it, you are playing against computers... Here, most people play online with each other, and the game plan is surprisingly simple: level up your hero (heroes) faster, gather an army, and go kill the opponent. :) Read this section; there's a lot of interesting stuff here, as I can't tell you how to win in just a few words... Read the Reports for example, see how others play

I play for a long time, develop slowly, so I can somehow win only on XL maps; on all others, I don't have time to develop and the enemy usually wipes me out in the very first month.

And you should develop faster :) Here is a plan:
1st week — take crypts/dwarf homes, build the infamous 6th level in the citadel (not the municipality)
2nd week — take medusas/naga banks/Hive/conservatories, increase your stats
3rd week — take Dragon Utopia, increase your stats even more.
Strive to keep up, what else can I say...
And you can even wipe out the opponent in the first week if you try..

For example, I've only just built a 6th level monster dwelling, and the enemy is already charging at me with a huge army that has plenty of upgraded 7ths.

For example, the opponent builds the 6th level, while I'll set up 1-3 in the fort with an external dwelling of minotaurs and mushrooms and be happy. And on the second week, I'll gather angels and vivs, and my army will be no smaller than the opp's :) Your goal is not to amass an army, but to kill the opponent by any means

So, I would like to know how to act correctly so that at least in the first week I can capture all nearby mines without losses?

Mines are not a guarantee of success, and you can get by normally without them

And also I'd like to know how to properly distribute troops between several heroes? Usually I have only 2 heroes — one combat, the other collects various free trinkets scattered across the map, but I don't know how to correctly distribute the army between both. Naturally, this always has sad consequences — I go far from the city, an enemy appears right there, and I cannot provide a decent defense.

To start with, master the concept of a "chain". The simplest chain connects the main hero with their home Castle. That is, you will need several heroes positioned within one move of each other. Then the army can return to the castle on any given day. For example, you see that an enemy is close to the castle. You give the army to the nearest hero, who passes it further, and so on until the castle itself. And the main hero, now without an army, attacks some neutrals and runs away from them. They are bought out in the castle and meet the army :) I hope I'm writing clearly. Once you learn how to use a chain — it can perform other tasks, like clearing a respawn, scouting territory, etc.

But overall — play online, read Articles, look at saves, and you'll understand everything over time