A nice summary of why we should just take God at His word. "The Meaning of 'Day' in Genesis":
What can we conclude concerning the length of the 'days' of creation? The usage of the word 'day,' with a number, means a 24-hour period. The absence of the article does not alter that meaning. Further, the use of 'evening' and 'morning' indicates that normal time is meant in Genesis 1. God, Himself, said that the creation took only six days. We also must ask ourselves, did Moses and God deceive us by using the word 'day,' when it really was a long period of time? If our answer is yes, then we should not use the Bible for any of our beliefs. For, if God can deceive us concerning the events of creation, He might have done that in regards to the life, death, and resurrection of our Lord. The bottom line is that we then can have no confidence in God's Word, if the long-day view is held. It is far better to believe God at His Word, and take the creation days as 24-hour days.