I love my 6+ but I find it too bulky to run with. I can see spending $350+ for the Apple watch if it could track my run without carrying the 6+ along too, but I'm not reading anything about addressing this issue.
What do you mean by track your run? You can use the Apple Watch as pedometer, calorie counter and heart rate sensor without an iPhone present and then sync that up to the Health app when they reunite.
If you're looking to map out your run or get directions via GPS while doing it you'll need to bring your iPhone with you.