Moral Dilemma

Wow.
The aforementioned job interview has turned into an actual offer! And I feel a little guilty since I'm not really looking to leave my position here.

I have a pretty decent job, the pay is not bad — for a state agency — I don't hate or even dislike the majority of my co-workers, I have a kick ass office (formerly a hospital room) with a huge desk covered with all my techno-geek stuff & a great view of a river flowing by down below.
As jobs go, there's not too much stress on a regular basis, just the occasional blips of emergencies & rushes that come up, I would imagine, in most businesses.
And they call & make me an offer?! The other applicants must have sucked, huh? Or maybe I just talk a better game than I realized. I mean, they did call all the references & still were interested.
The job is a little more money, not a huge increase —near the bottom of the next higher pay schedule, but I really do think that it could be immensely over my head.
And I know that the people offering me this new position are not looking for someone short term, they're just entering the second phase of a long project that will, in all likelihood, take two or three years to complete. And I would be a real schmuck to go over there for just a few months, & then leave on my own venture just when I was settling in to their routine.
I really feel that I shouldn't take their offer, & am still amazed that I was the top pick. Hell, I was just trying to get out there and put my face around a little, ya know?
Should I feel guilty about not taking their offer? I certainly don't want to leave a bad impression with them. But, as is often the case, I think, I don't want to bolt from something secure that's not too bad a thing.