If your main concern is a running watch, then don't get the Apple or the Fitbit. they are not full functioned running watches. I am a Garmin Fanboy so my opinion is admittedly biased. Garmin and TomTom have the best full-featured running watches. If you have the budget for an Apple Watch then you can afford a Garmin 645 Music. Which is what I would buy. To be able to leave my iPhone home has a lot of appeal to me.
The TomTom Spark and the Garmin 135 are more budget priced - but still have a lot of good features.
Maybe the most important feature you need to look for is to be able to program complex intervals into your watch. And I could be wrong about it but I think that feature is only available on full-featured running watches.
although now that I think about it - many iPhone apps can do that all by themselves. hmmm. Heart rate monitoring is a nice tool but somewhat overrated. Once I learned what my "zones" felt like, i really didn't worry about checking my HR. Maybe you can get by with just your iPhone for now. your iPhone and a good running app. But if you are like me and wish to leave the iPhone home, then the 645 or the Spark music might be the way to go.
Oh, wait, just looked up who I was talking to. Hi Rylie! Maybe you don't want to ditch the iPhone. A phone is a good safety feature for a runner. you can call a friend to pick you up when you can't go any further. So forget everything I just said and just use a good running app. one that allows you to program intervals. I use RunKeeper but I think MyFitnessPal has the same features.
Jeff Galloway has written the book on half marathon running. No, really. He has literally written the book “Half-Marathon: You Can Do It”. Take it out of the library or buy it at a bookstore.
Speaking of books, while you're at the library pick up a copy of Chi-Running by Danny Dreyer. His techniques and insight will serve you well. And I just discovered he has a new book called Chi-running Marathon.
Galloway has a pretty good website with training plans. I've used his training plans before and I like them because they are general enough that they can be used regardless of your fitness level. But get the book. The info on nutrition and preparation is important.
Try to run at least one 5 K in the meantime. The experience is important. Plus you will learn what to expect. And what to do for race preparation. Try to get in a 10 K if possible.
I don't know about you but I get bored running long distances and I like to listen to podcasts. If you're like me you are going to want to check out the Marathon Training Academy and the Runner's World Half Marathon Challenge
While I was getting those links for you I noticed an app for running a half marathon and a training app by Jeff Galloway