I've never run a game of this length. Shorter and longer but not this length. I know, not helpful.
My first thought is look at most of the action adventure movies released over the past 20 years and a lot of them have plots that involve a known event that will occur in about a week.
So I say session one should involve revealing the whole basic plot in the first session
"OK, the presidents speech will be in California in 1 week. You guys have to find the ...what did you call it... half/Fiend Amish Psychic vampire??? before then or he will lose all the electoral votes from California. Got it?"
In the mean time Brother Ismael Hellfire is busy tracking the spent nuclear fuel shipment through the country so he can spike the puch at the primary....Sorry I'm on a sugar high.
back to a point...
Use week 1 as exposition-establish characters, major NPCs and support personalities, the bad guy and the major plot.
Week two-looking for a solution. The soultion should have two to three possible answers, 1 is relatively easy an sort of obvious, 2 should be really obvious but so wrong and dangerous only the truly chaotic evil would try it, 3 is not obvious and is incredibly hard but will work.
Week three- First attempt at getting job done, most likely the obvious solution. should end in failure. Most likely because the bad guy thought of it first and is covering his arse. This will force them to week four where....
Week four-the twist. No modern day plot is worth ink (or dice) without a plot twist. The plot twist is usually even better if it shows that the earlier easy solution would not have worked anyway and the ultra hard dangerous solution was the only real answer.
Week Five-showdown in little china (or California)!!!!!!!!!!
Now this all seems very obvious and very cliche. Yes, that is why hollywood is full of very rich people. Cliche works and under a strict five week game schedule cliche is the only real thing you have to work with.
Besides, you and your players have probably been playing for a long time and everyone needs a trip down cliche lane every once in a while.
I know I'm not providing you with alot here but this is the only way I could think to get my mind around the particulars.
Let us know how it works?????