In fact, if you have hours ahead of you, it can be daunting. The expectation is high. You feel you must produce thousands of words and therefore immediately put yourself under the cosh. Writing shouldn't be like that. When you have hours, you can relax while you warm up and then enjoy the excitement and tension that slowly builds as your story takes shape. So, love the long time you have, but respect it. It is a luxury.
I have mentioned before that we all have our own framework of time in which we can write before we break off for a while. However, just because the time available is less than our perfect quantity doesn't mean it shouldn't be used. After all, something might crop up later in the day to prevent any further writing taking place. Then you will be relieved you used that gratuitous few minutes to advance your story/do some research/edit. It's never wasted.