Posture is key. I've found that keeping my core engaged and my pelvis aligned has made a huge difference in my efficiency. It takes practice, but it's worth it. Also, try to land more midfoot, it'll help reduce the impact on your joints.
I've seen improvements in VO2 max with hill repeats and tempo runs. Focus on pushing yourself outside of your comfort zone.
Yeah, endurance is key for a half marathon. I did my first one a year ago and I wish I'd focused more on building my mileage gradually. Don't try to do too much too soon, and listen to your body. Rest days are crucial. Also, don't neglect your nutrition and hydration, it makes a big difference on race day.
7-7.5 hours is a good range. I've found that 7 hours is where I feel most rested. I've also started using earplugs, it's amazing how much of a difference it makes. I've also been trying to establish a consistent sleep schedule, even on weekends. It's not easy, but it's worth it.