Decisons, Decisions

My back is stiff and sore. My knees are stiff and sore. My hands are stiff and sore. My wrists are sore. I've woken up several nights recently with cramps in my hamstrings so severe that my wife has had to massage them; I never had these kinds of cramps before. I feel like I've aged years in just two months. Much of my job feels like torture with no prospect of relief. I'm not getting any faster at doing what I started doing two months ago, and, now that I'm being given more to do, I'm staying an hour to an hour-and-a-half later than everyone else to complete my tasks.

I want to find another steady job I can do better and that pays benefits and doesn't destroy my body, but I need to upgrade my typing and computer skills to improve my chances of getting and keeping it, yet, I have no time to develop these skills, if I even can develop them, as long as I spend so much time each day at this job. I'm sure it's only a matter of time before I'm let go from underproduction and inability to learn the new tasks that I'll certainly be asked to perform in time, but I'm wondering if I should quit before that happens and start full-time preparing and looking for another job, or if it would be better to hang on and endure the torment until I'm asked to leave. And if I quit first, how should I do it? What should I tell and not tell my boss about the reason for my decision?

Yet, even after I leave from resigning or being fired, what job do I seek? What decent job can I, at my age and with my sparse background and all my learning impediments and slowness, reasonably expect to be hired for and to be able to keep?

It would be easy to surrender to total despair, but I'm trying to keep hope alive and make the right decisions and follow through with them.