Maybe you can't make the whole body comfortable, but make at least part of the body comfortable and stay with that part. As for the pains, let them b… - Ṭhānissaro Bhikkhu

There’s a passage where [the Buddha] contrasts his way of teaching with what he calls training in bombast. Training in bombast is where you’re taught things that are very poetic, that sound very high, very lovely, very inspiring, but no one is encouraged to ask what, precisely, they mean. After all, in bombast there really is no precise meaning. It’s all just vague, high-sounding words. But, as the Buddha said, he taught cross-questioning. Your training with him was in cross-questioning. When there was a teaching you didn’t understand, he encouraged you to ask, “What’s the meaning of this? What’s the purpose of that? How far should this word be taken?” That way, wherever there are any doubts or uncertainties, you can clear them up.

Society tends to slough off the problems of aging, illness, and death, tends to push them off to the side because other things seem more pressing. Making a lot of money is more important. Having fulfilling relationships is more important. Whatever. And the big issues in life — the fact that you're headed for the sufferings and indignities that come with an aging, ill, or dying body — get pushed off, pushed out of the way. "Not yet, not yet, maybe some other time." And of course when that other time does arrive and these things come barging in, they won't accept your "not yet," won't be pushed out anymore. If you haven't prepared yourself for them, you'll really be up the creek, at a total loss.

Ardently alert means that when the mind is staying with the breath, you try to be as sensitive as possible in adjusting it to make it feel good, and in monitoring the results of your efforts. Try long breathing to see how it feels. Try short breathing, heavy breathing, light breathing, deep, shallow. The more refined you can make your awareness, the better the meditation goes because you can make the breath more and more refined, a more and more comfortable place for the mind to stay. Then you can let that sense of comfort spread throughout the body. Think of the breath not simply as the air coming in and out the lungs, but as the flow of energy throughout the whole body. The more refined your awareness, the more sensitive you can be to that flow. The more sensitive you are, the more refined the breath becomes, the more gratifying, the more absorbing it becomes as a place to stay.