I ran "Shackled City" from start to finish a few years ago. (Note: the magazine version, which has one fewer chapter than the compiled hardback version.) It took eleven real-world months from start to finish, playing almost every week for 6ish hours a week.
We also tried "Savage Tide", but this one fell apart after just a few sessions, mostly due to real-world scheduling problems. (There was also an issue that ST seemed much more brutal than SC, at least right at the start.)
The first and most important piece of advice I would offer DMs wanting to run an Adventure Path is this: make the campaign your own. If you run the campaign strictly by-the-book, you're likely to find (as we did) that the characters tend to be more than a little one-dimensional. Since the campaign is basically mapped out, there's not really much scope for the PCs to follow their own agendas and investigations unless those happen to match up with the campaign as written. It's basically up the DM to fill this in - I recommend using the AP as the spine of your campaign, and bolstering it with additional encounters, side-treks and role-playing to fill it out and make the world and the PCs live.
The second thing I would recommend is: be flexible. Sure, "Rise of the Runelords" is a six-part campaign taking PCs from 1st to 15th level, and it would seem to be a real shame to throw out five of those parts when the players decide they instead want to dedicate their careers to hunting down crazy goblins, but... isn't it better to have a really great campaign doing that, then try to force people into a path they don't really want? (And, besides, those other five parts can still be used later; just file off the serial numbers.)
Thirdly, and contrary to some advice higher in the thread, I would recommend being honest and upfront about the fact that this is a pregenerated Adventure Path. Further, I would advise giving the players at least some heads-up of what to expect, so they can build characters that will be fun to play. A large part of "Age of Worms" is Rogue-unfriendly, while at least one chapter of "Second Darkness" is troublesome for Paladins... better to tell the players ahead of time, and let them make informed choices.
Fourthly, and this may be a purely personal thing, I would recommend only running an Adventure Path if your group can dedicate the time to get together quite regularly to play through it. One of the major reasons "Savage Tide" failed for us was that we had a break of about a month, during which any interest in the campaign died. Conversely, "Shackled City" worked really well, in part because we missed very few weeks. Unfortunately, while I would dearly love to run "Age of Worms" (in Dark Sun), "Curse of the Crimson Throne" and "Legacy of Fire", my group will almost certainly never play through any of these, because our real-world commitments just don't allow it any longer.
Fifthly, I would note that the Paizo Adventure Paths are really quite brutal. I would recommend ensuring you have at least six characters in play. Further, it might be a good idea to have each player run two characters, because otherwise they may spend quite a lot of time out of the game due to character death. YMMV, of course.
Oh, and finally: running a pre-generated Adventure Path can save an awful lot of prep time, especially at high level. However, it is certainly not a means to skip out of preparation altogether! Especially at high level, the DM really needs to spend a couple of hours each week thinking through encounters, tactics and powers for the bad guys. (Which is still an awful lot better than doing that and several more hours generating the stat-blocks and encounter locations from scratch.)
I hope some of that helps.
