God called the light Day, and the darkness he called Night. And there was evening and there was morning, the first day.
If there was ever any debate over the literal-ness of the twenty-four hour days in Genesis, this verse should halt the debate. It is abundantly obvious that the day that is described here is the result of the light and the dark, just like a day today. If these days were long time periods, then each time period would be encompassed by half light and half dark, which doesn't make any sense.
The real question to be asked about Genesis is not the length of the days, or any other debate for that matter, but whether or not you believe it to be God's word. That is the real question to ask. Trying to mold Genesis into some strange evolutionary model is not right, but it is also not necessary! The Biblical model works without any extra explaining. In fact, it works even better than any evolutionary model.
One little side note that I would like to point out: Notice that this first day of creation has a day and a night, but there is no sun yet. This, by the way would not fit into any evolutionary models, but works great in the Biblical one. It does leave me with some questions though.
What did the morning look like?
Did this light bring warmth?
Where was the light coming from?
Was it just coming from God?