Consider longer sets as taking your audience on a journey! You don't need to play filler tracks if you have more good tracks.
You looking at the time makes me think you're not comfortable with the situation. The fear about messing up is the single biggest obstacle you need to overcome. Being in the comfort-zone will make a lot things easier and more enjoyable for now.
It seems to me that you're playing at Top 40-clubs. Am I right? People tend to loose interest quickly in these when you don't play tracks which they recognize. So why not dig out some classics they wouldn't expect?
Also why not go to a club where similar music is played and see what the DJs there do for a full evening? I used to wonder how that works in Indie-clubs until I was there for a few evenings.
You could also look at
http://serato.com/playlists for hints on songs you can play but going to a club to catch the mood is a more valuable experience in my opinion.
If you already played a track there a ways of brinking it back like a remix or mashing it up with some other songs. Having certain songs or parts of it re-appear every now and then during sets is actually one of my favourite parts.
Have some crates sorted by criteria like mood or genre. Then determine which track to play according to the situation.
Don't freak out when you empty the dancefloor! It happens every now and then. Also keep in mind the
time-line of a dancefloor!
Here are some Digital DJ Tips-links which may help you:
Bookmarks