This is how I gamed for nearly six years, in a 4 to 5 hour block.
First, you need them to get down to gaming immediately. We only gamed for 4 hours, we hung out for 30 mins on either side of the game. So I want you to have a "Starting Bell". It can literally be a bell, or buzzer, or it could just be a phrase. AT that point, people know they need to get it together and game.
Second: Pun Tax/ Joke Tax/ Distraction Tax. This should be real money or you can dock them experience points. However if you are docking people things, you need to give them a tangible badge of shame (taking away bonus ex tokens, a yellow sticky that says Tax'd and so on). Anyone taking away from the game must pony up.
Third: Think of your adventure as a One Hour TV show (and think Babylon 5 or StarGate and how they handle large story arcs as expressed in single episodes). You have one main dramatic plot line that entangles everyone and one subplot that entangles and spot lights one (maybe two) characters.
- Script it like a TV show. Big Bang Intro, First plot complication, Second Plot complication, Dramatic Ending. (you have one hour per 15 minutes of TV "air time"
- Keep the story and interaction and simple and clean.
- Keep the game going with narration at all time. If there is dead air, something is wrong.
- Keep the Silver Rule going: If it does not seem like a TV episode, you are doing something wrong.
Also bang it into your players head that they are running in a One Hour TV format. So they will start applying the same rules to their own play and understand what you are doing.
Fourth: See the [Running a Game, the MoonHunter way] @ http://www.strolen.com/content.php?node=1462
Fourth and a half: As listed above: No book keeping during the game. Ever. Generate those characters and all that paperwork before the game. Pregenerate the characters if you have to. If this is an ongoing once a week sort of game, you can spend one session generating characters and doing "tiny roleplay", but I doubt you will get together that often
Fifth: Combat is the major time killer.
[Running Tactical Scenes at Lightning Speeds with Many Players]http://www.strolen.com/content.php?node=1794
(I might also recomend combat sheets or cards with your initiative, hit/ dodge/ parry/ armor/ penalty rolls and mods plus damage be made up).
In short game runs, you, the GM, need to be in control and pushing the game along.