Negative splits are a great approach. I've also found it helpful to focus on a consistent effort rather than a specific pace. And I agree, don't be afraid to walk if you need to, it's all about listening to your body. As for fueling, I'm a fan of keeping it simple and sticking to what you know works for you. Good luck with your long runs, and don't be afraid to mix it up and try new things to find what works best for you.
Leg swings and hip circles are great choices. I also like to add some arm circles and torso twists to get my whole body moving. It's amazing how much of a difference it can make in how you feel during the run. Plus, it's a good way to get your heart rate up without exhausting yourself.