My goal with both of my daughters was to have them driving anywhere and everywhere as soon as safely possible. This meant LOTS of practice - they were both behind the wheel as often as possible as soon as they had their permits at age 15. That meant they drove anywhere I needed to go - to the grocery store, to school, after school for errands, etc.
Once they had enough experience for me to feel comfortable, they graduated to interstate driving. First on Sunday mornings to a neighboring town about an hour away with a good mall, then several trips to the same mall on the weekends to practice interstate drivng, etc. I also let them each drive on the highway with me several times back and forth to the same area or on trips to Florida or other vacation driving. Again, practice, practice, practice was my goal.
DD23 went to college about 3 hours away and her first long distance drive was to her college during her senior year of high school. Once she had successfully navigated that, she did the drive a couple more times before she started her freshman year. By that point, I felt comfortable with her driving long distances and she was able to drive back and forth to school, the beach, the mountains, etc. She and some friends drove to Washington, DC and to Orlando and Miami during the course of her college years. Now she goes wherever she needs to, often driving 6 - 8 hours at a stretch for work-related travel.
My younger daughter benefited from my experience with her older sister and we got her out on the road as soon as possible. Mostly because she was my "baby" and I was sure she would never be able to drive on the highway like her older sister - I knew I had to get over my fear for her sake. So out we went, and she drove back and forth to her friends' houses in a town about an hour away, to the same mall her sister had visited, etc. I just made myself give her as much highway driving experience as possible. Her first long haul was to visit her sister in college - she had driven it before with me in the car but it was a nerve-wracking day when she did it for the first time by herself!! Now she has done it several times and I think she can drive anywhere, anytime - but I do think if she needed to drive more than 4 - 5 hours I would ask her to split it up into two days. She gets tired and bored at that point. She will be going to college about 12 hours away so when she drives her car home it will be a two day event.
Good luck, OP!! It is nerve wracking for sure but once I got to a certain point I realized they were good and careful drivers and I was able to let them drive on their own with only the normal worry anyone has about loved ones on the highway.
Though my friend who was 18 was supposed to be driving my car since it was illegal for me to be driving after midnight. However, I had 8 more months driving experience and she was scaring the crap out of me and another friend so I took the calculated risk. 
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