I'm a fairly new to Shadowrun. I began as a player, and after that campaign fell apart with half the team joining the army or moving to Texas, we decided to start a new campaign. This time I'm the GM. We've had 2 runs (over 3 sessions), all very fun and successful. I have used the runs from the SR overlords, and find that they're a bit lacking. Maybe it's just my ability as GM, but...

Well, I started writing a run. It's straightforward. Mr Johnson wants a rival killed, which allows him to obtain a large sum of money. However this rival is a very nice guy that has done nothing wrong. Mostly a moral choice.

It will end with A.) the runners siding with Mr Johnson, and getting screwed out of most of their money (after a nasty fight), and notoriety or B.) them siding with the rival getting comparatively more money and getting jumped by Mr Johnson and his thugs.

Seems all fine and dandy, but I am new to this whole thing, and I have a sneaking suspicion that this run will fall short. I don't want a 12 hour run, and I don't want a 2 hour run. 4 hours seems about the right amount of time.

So the run is basically:
1.) meet Johnson
2.) Legwork
3.) find rival
4.) moral decision
5.) wrap it up after a fight.

I can send the text that I have written out. Any input? Advice? Experience? Should I just use a different pre made run (I do have a good one ready, which will be next if I use this one this coming Saturday)?

Thank you, any input is appreciated.

[disclaimer] I've never written an adventure for Shadowrun Missions, only for home games.

Standard advice from writing con modules - playtest! You need to run the game with at least one group you've never run before, and then after tweaking have it run by a GM who doesn't know your style (possibly from your first playtest group).
Also, be careful with the 'get screwed over by Johnson' plot. It got down to death in 1st ed and generally gets the result of the PCs simply ignoring everything else to kill him, his associates, family, friends and pets.

Looking forward to what the people who write the Shadowrun Missions have to say...

I have written for Shadowrun Missions, but I don't think PenCapBlue is talking about Shadowrun Missions, but rather Shadowrun missions, which is to say adventures. If this is the case, I've got the following advice.

Start with published (also called "canned") adventures. The first task of writers of any stripe is to read the kind of material they intend to write. Speaking of Shadowrun Missions, there are gobs of free adventures at the Shadowrun Missions download site that you can either use as reference material or actually run in your games; I suggest the latter, because it will give you a better feel for how adventures come together.

There are also a few adventures available at my site, which is linked in my sig.

Just sticking my nose in here.

I think Aaron's correct in that PenCapBlue isn't looking to write an adventure for the Missions campaign. (Obviously, we wouldn't be able to get one through the writing system and approved by Saturday. ) Having said that, the campaign is always open to and looking for new authors. (If you're interested, drop me a PM for more info.)

Beyond that, I think Aaron's covered the key points, though Whitelaughter's point about playtesting for public games is a crucial one.