Myopia. The most common refractive eye problem. Caused by light focusing in front of the retina (sensory part of the eye). The light focuses too early. This is usually caused by the front part of the eye (cornea) and/or the natural lens of the eye being too powerful compared to the length of the eye. Can also be caused by having normal power of those components but too long eye.
Glasses would have minus lenses, which decrease the power, so the light will focus a bit further back, right on the retina.
Ciliary muscle (focusing). Has the ability to contract and relax. Innervated by the parasympathetic nervous system (not voluntary system). Smooth muscle. Cannot be strengthened by causing muscle growth any more than could your intestine. When you look up close, you need more power to focus things. This is just an optical fact. Things up close need more power to be seen clearly by the eye. The ciliary muscle provides this by contracting and causing the lens in your eye to change shape, giving more power.
When you look far away, you need less power. So the muscle relaxes. This is why you don't use the ciliary muscle in the distance. At the maximum state of relaxation (i.e. the power 'added' by the muscle is zero), the case may be that the front of the eye (whose shape stays constant more or less) is providing too much power still. Or the eye is too long. So the light is still focusing in front of the retina. Now what do you do? The ciliary muscle is totally relaxed. But you have the light overfocused still. The only solution is to put on minus lenses to decrease the power/focus.
It doesn't matter whether you use glasses or not. If you don't, you just won't see clearly. Doesn't mean your refractive error will get worse. If you wear the glasses, your refractive error will still get worse. If you wear a lens on one eye and not the other, the change in refractive error over time will occur the same in both eyes, if a change will occur. In myopes, they usually get a bit more myopic in their teens and early 20s. People who don't need glasses at the distance mostly stay like that over time.

Presbyopia. Loss of the ability to focus with age. Happens to 100% of people. Average age: 43. Doesn't matter if you wear glasses or not. Doesn't matter if you are a hunter and don't read. Doesn't matter if you live in a cubicle and stare at the computer all day. Doesn't matter if you do eye exercises all day. The only thing that exercises can help you do is help the muscle change between contracting and relaxing. People who spend too much time doing one or the other (usually too much time contracting), can get trouble going into the other mode (usually relaxing). But this usually doesn't persist after they stop that intense activity. That's all those exercises can help you with. You will still lose your ability to focus in your 40s.

I didn't really go into detail about the accomodation to keep the discussion more simple, but I will coz it's relevant. The lens in your eye is attached to the ciliary muscle by little fibers called zonules. When your muscle contracts it bunches up and decreases the distance between the muscle and the lens. So the fibers can relax, which causes the lens to take a more ball-like shape, which has more power. This happens when you look at things up close.
When you look in the distance, the muscle relaxes, which causes it to get flatter and farther away from the lens, which causes tension in the fibers to increase, causing the lens to be pulled into a flatter shape, which has less power. The loss of ability to get the lens to the more ball-like configuration with more power is presbyopia. Since you can't get it to give you more power, you need glasses to do it for you. That's why you wear reading glasses.
This loss of ability may not be solely due to the muscle. It could also be due to changes in the fibers and the lens becoming more stiff with age, so you can't get it to ball up anymore even though the tension is relaxed by the muscle contracting (which decreases tension on the fibers). Whatever way you choose to explain the loss, exercising the muscle won't help you.
The only way you'll avoid reading glasses is if you are myopic and things are already overfocused for the distance, so when you have no glasses on and you're looking at something close up (which is when you need more focusing power), the focus will be right because of your myopia. But then you won't see things in the distance clear coz the myopia causes those things to be overfocused. Some people may say they see just fine in the distance too even though they're myopic. But if you test them with an acuity chart, they will not get 20/20.

Hope that clears up things more than causes confusion.