I think you should take a bunch of short hikes first. Starting with a 17 day hike is just asking for trouble. 15 miles a day isn't too agressive, but if you're inexperienced it should be worked up to. At the very least, you need some miles to let your feet and hiking boots adapt to each other, and if they don't you need to get different boots. Rather painful if you find out you've got the wrong boots 4 days into the middle of nowhere! Same goes for the rest of your gear. If you can't take some break-in hikes, make sure you at least have a few "outs" in the trip in case things aren't going as planned.

If it's a trail where it's easy to get lost, I would suggest a GPS. You don't need to keep it on all the time, just turn it on every once in awhile for a few minutes to get a fix when you're not absolutely sure where you are. But if you want to bring along enough batteries for the trip, it's nice to see a track log and use it to measure your speed and distance.