How long did Light's father have to live? Since Light's father took the shinigami eyes, which reduce your lifespan by 1/2, and died the same day, does this mean he only had 2 days left to live?
How to Use: XLII (from Volume 08):
  
  
  The use of the DEATH NOTE in the human world sometimes affects other human’s lives or shortens their original life-span, even though
  their names are not actually written in the DEATH NOTE itself. In
  these cases, no matter the cause, the god of death sees only the
  original life-span and not the shortened life-span.
  


With this, I would say Soichiro had some time to live still, but the use of the death note made his death unnatural.