My check engine light

I have a 1990 legacy sedan with 175,000 miles on it. A few months ago the check engine light started to come on after the car had been driven for at least 10 minutes, then had been stopped and restarted. But, after a sufficient amount of down time, about 6 hours, the light would be off when i went to drive again. Since yesterday, the light has been on continously. The car runs relatively well, it is a little rough at starting and when I shut it off it just seems like the car is giving up. The engine runs slightly labored, but that is no different from when the light is off. It seems to be a product of the car's age. Does anyone have any idea why this light is now on and seems not to be giving up?