So when I spectated the Princess Half a couple of years ago, the three things I found to be most helpful in finding my BFF were my own knowledge of the race course, my own knowledge of WDW's transportation system, and being able to text with her to figure out where she was.
They do have runner tracking that you can sign up for to get text alerts...most of the time this works really well, but sometimes there can be a delay - that was the case when we were trying to see my BFF. There was a big delay with the texts and we didn't get timely updates for her (they send updates at the start, 5K, 10K, 15K and finish for half marathons, if I remember correctly). But she had her phone on her and was texting us to let us know where she was, so we knew when to look for her coming into the Magic Kingdom.
Here is last year's event guide so you can check out the course map:
http://as1.wdpromedia.com/media/rundisney/pdf/princess/2016/2016-princess-race-weekend-program.pdf (since I don't anticipate 2017's event guide coming out for another few weeks...)
The courses should be exactly the same, if not very similar this year. You can see that the Magic Kingdom portion of the course comes in between Mile Markers 5 and 6 - plenty of time for you to see her at Main Street and the finish (forgot to mention in my previous post - you WILL NOT need to use a ticket to see her in Magic Kingdom). Pages 52-54 of the Event Guide linked above have information on the ChEAR Squad and spectator viewpoints.