Help! My Subaru engine is scary slow, and the check engine light is on

I have an older model manual transmition Subaru Impreza, and I'm having
some troubles.  My car was little shaky on ignition and when I put it in gear it
was going max speed around 4 miles an hour.   It shifted alright and their were
no suspicious sounds, but it seemed like the engine wasn't getting much gas.  
The check engine light was on, but I had no previous warning lights and the
car had driven fine the day before.  I checked inside and my oil was low and
pretty dirty.  I think it had been leaking out of the block slowly as older cars
tend to do. Is this a preamble to my engine seizing or some other problem?