Life and Death

I've been thinking a lot lately about life and death. While I don't believe that God wants to do us harm I am convinced that He has numbered our days. He has given us so many days to live and then comes our death. How we die is dependent on our decisions while we live, but when we die is up to God.
I struggle because this means that God has only given some a few days to live and others he has given years. I know that there are passages of Scripture concerning this issue, but at the moment I am only speaking out of my own thoughts, and ideas. If the number of our days is predetermined it also means that we can not add years to our lives by our lifestyle choices, but that we merely improve the quality of the life we have to live. It also means that we can not take years away, at least not literally but our decisions can chisel away years of productivity, and the 'abundant life' that God designed for us to live.
Morbid thoughts I know but I am being challenged to reevaluate my day to day decisions because of this. I face eternity and must be prepared for it at any moment. I must be prepared for the fact that eternity means more than just my destination but the kind of welcome I will receive is a part of my eternal destiny too.