Go to the adventure path message boards there is one specifically for Runelords and will have lots of great discussion, by chapter, from lots of other GMs who have run the adventure before you!
My advice for a first time GM is, read ahead as you want to know where the story leads in case your players go a-wandering. It's also interesting to read the Obituary log to see where a lot of groups end up running afoul of the bad guys.
Good luck!
Hardest part for me was learning to let things go.
So you spent hours preparing for a fight assuming the party would come in through the front. But instead they came in through the back unnoticed through an entrance you thought impossible to find since the DC is near impossible. But you forgot about that item that lets them detect secret doors that you gave them eight sessions ago. Now what?
So you put an alarm trap in at the front gate expecting the party rogue to find it, but for the first time in his entire career he throws caution to the wind and walks right over it. Now the entire fortress is aware of the enemy threat, and the party is standing right where those arrow slits and murder holes are best used. Now what?
So you spent time learning all the nasty tricks your big boss succubus can pull on the party, but the cleric makes a knowledge check before the fight based on some obscure symbol you placed in the dungeon more for flavor than anything and proceedes to cast protection from evil on everyone before they engage. A few unlucky rolls later and the big bad evil boss is down without a memorable fight. Now what?
Let it go. Let the party win when they think and plan ahead. Let the monsters whoop on them when they don't. Your job is not to make sure the final fight is a nail-bitter. Nor is it to make sure the party makes it there intact. Make sure your party knows that you will be playing the game straight. Cleverness will lead to smashing success. Foolhardiness will lead to situations the party may have to run from. And if they refuse to run, they may die (some or all). And your role is not to prevent the good or the bad, but to let go and let it happen.
Sounds simple, but it was the hardest thing to learn as a DM for me.
First off: Congratulations on taking up the GM Mantle!
Second: I'll echo the advice to go check out the Runelords subforum on the AP message board here. There's an amazing amount of excellent advice there.
Third: The first rule of any RPG is Have FUN! As long as the players AND you are having fun, then you are doing it right.
Fourth: While I absolutely agree with reading the entire campaign ahead of time (preferably you should finish all 6 books prior to the first session) keep in mind that you may need to be flexible. The unwritten rule of RPGs is that no plan, no matter how carefully constructed or cleverly conceived, ever survives first contact with the PCs. This is largely an echo of what Lacdannan was getting at.
However, it goes deeper than that. The books do a great job of seeding treasure rewards into the campaign so that, in theory, you as the GM never have to worry about it. However, no amount of pre-written treasure parcels can ever fully conceive of what type of party your players come up with. While there is some fairly mage-friendly loot in there, if your players all decide to play mages, they might start feeling the pinch. Granted, its not likely that everyone will play a mage, but the number of possible party compositions are nearly endless, so you may find that you need to occasionally alter a treasure parcel, or at least open up the stores to allow for more purchasable items. While I think from a player's standpoint that learning to use what you find is a large part of the fun, that's a) my personal opinion, and b) still doesn't mean that you should never throw a player a bone. At the same time, don't feel the need to simply toss in everything that each player wants for his or her character. Adaptability is a big part of the fun.