Well it's been mentioned in a few threads, some people moved Rannick closer to Sandpoint and loved it. I personally think that the travel is an interesting aspect to the story. A cool idea someone had was to use enchanted forests around turtleback to travel back to sandpoint area easily. If you did this you could alter the ending of Hook and have them rush to the aid of Sandpoint right at the end of the adventure.
First off, over the next little bit, use down time in the game to talk about the other areas in Varisa, make people think about life outside the town, it'll make people more willing to go if you already excited them about other places. Go ahead and mention the Ogres in Hook Mountain, it'll be many sessions before they ever have to go there.
What's the background of your players? Do you think you can come up with ideas on how to get them personally involved with any of the plots, such as family in Magnimar, or somesuch?
One of the things I want to do the next time I run Skinsaw is alter the events a little. Have less information for the party during the first murder, make it so they can't figure it out until a few murders in. Maybe a cool chase scene as they foul an attempted murder, something. It'll hopefully make tracking down the skinsaw man that much more important, and keeps you in Sandpoint a little longer. Also, I'm going to find a way to logically have the skinsaw man speak to the party while they're in the manor, some sort of disembodied voice where certain rooms get different personalities. It'll give you time to let slip any info about magnimar and makes the house more player involved, someone just wrote that they felt that it was just a bunch of traps and their players hated it.
I mentioned this to someone else who had troubles with this section, play up the connections between the murder victims, maybe the skinsaw man starts leaving a list of who he's killed behind or even tells you who his next victim is. The point is that Skinsaw is targeting people. And the players know this. But they don't know why he's going after these particular people. Then when they find the note (or alter it and leave a list of people to be killed off in the house) they will hopefully care that Skinsaw was sent by and working for someone else.
If this isn't enough to get them to go to Magnimar, don't force it. Let a few days go by and then send another killer out, this time just a skinsaw cultist. Or have someone come down and ask for help. The two cities are close enough that if they had similiar killings going on they might know about this. The Sheriff tells the party that this is just the tip of the iceburg as it were. If that's not enough, do a little side quest, something like Kobold King and jump to Hook Mountain.
You know your players better than anyone, what kind of stories do they like? If sending them to Tuttleback on official business isn't good enough, what about some NPC they like and trust asking them to help them make the trip to sell something or the like. You could use (female elven ranger, tired can't remember how to spell name) and play up her father, or cut that out entirely and just use a merchant. Or maybe if you have a ranger in the party, they once spent time with the Black Arrows, heck alter the story a bit and have one the PC's father stationed at Rannick. You've got the time to set that up.
If the players get into Skinsaw and you think you cna pull it off, how about trying to place evidence that the two Lamia Matriarchs are in communication somehow and the party finds out that the town is in danger. That way its a direct tie in and not two missions.
Once you've done that, the rest of the story goes pretty smoothly, except finding the runeforge, but that's a while down the line. I'm tired, can't think of anything else to say.
Oh, here's one thing.
They only have to think they know what's going on, hopefully you can get them to think that Mokmurian is the big boss, and then pull out the Runelord after they defeated him.
Do what you would do for a mystery adventure. Put all the clues to the identity of the BBEG on index cards with HOW the PCs would find them out. Then, write out a clue hammer to use in case they miss it, for example a certain expert with certain fringe theories.
Also, if Fort Rannick is nearby, you have to explain why the PCs are needed to save the day in #1 at the lair without military aid and why in #2 it's them scouring the countryside and not the soldiers.