I also believe in a created universe that is a rather old place. I think Genesis does a pretty good job of explaining the big bang and subsequent events. Whether the time frame is interpreted as you state or viewed allegorically isn't terribly important to me. God wanted Moses to explain origins to people a few thousand years ago in a way they would understand. He wanted them to know that creation was a long and hard process, even for the omnipotent God. Had Moses brought back a detailed explanation of the big bang, quantum mechanics and the rest, the Hebrews probably would have gone back to worshiping statues of cows.
It's interesting to me that William Jennings Bryan was an old earth believer and that the scene from Inherit the Wind where he is broken on the witness stand for young earth views never happened in the Scopes trial. The calculation of a young earth creation date was done by one Bishop Ussher in the seventeenth century and is not in the Bible. Since some of my ancestors are Anglo-Irish Quakers who Bishop's Ussher's anglican ilk persecuted, I've never thought of his calculations as authoritative.