I've used the apple watch, and iphone and AW alone outdoors and in the end they result in about the same mileage, within about .1 over 4-5 miles, but how they GET there seems to be different. I find when running with just the AW2, my pace is totally off. There is certainly real time sampling error with the AW alone, but the start and end points and many points in between help it self correct AFTER the fact. That is how I get the same result when using both or just AW (or of course iphone alone). So, run with your AW and iphone if you can for the first few runs (you probably already have) and then you can probably do with just the AW and some headphones.
As others have said, its really about the kitchen so focus on calories IN as well, and don't auto adjust just because you are running. One doesn't want TOO substantial a deficit but targeting 15-20% below daily RMR/BMR and letting the running add to that 3-5 days a week is going to get you the best results. Make sure to each SOME carbs, don't carb down too much as running is going to require ready available energy, but get good carbs prior to running. Also, I have found that contrary to what health blogs are saying these days I DON'T refuel right after running but wait at least an hour before adding protein and some carbs back in.
Also, running at pace isn't as good as shorter bouts of running / sprinter combos for actually LESS time. If you're looking for true cardio conditioning, then some of that HIIT mixed with the steady state is going to be great, but HIIT is going to be better for Fat loss.
Also, if weight loss is part of your goal, trying to do SOMETHING in the morning 15-20 minutes and then something more substantial in the afternoon/evening is going to get you better results.