@bellsmom It's a lot worse right up until you leave.
We do ~500 mile drive (one way) with our 3 boys (6, 4, and 2 now) about 2.5 times a year to see my family.
We've done this drive with kids at least 15 times, maybe more.
You have an urge to get there as quickly as possible, but it has to be tempered with driving safely and knowing that the kids need time to stretch and rest.
12 hours is a lot to do in one shot, but it goes by quickly. I know my own limits when driving, I know when I need water, caffeine, or a different audio source. Don't push yourself beyond your abilities and don't be ashamed to admit you need to take a break or split the drive.
Finally, make it fun! Queue up audio books, podcasts, or playlists you want to listen to. Talk to your SO about life stuff, work stuff, or stuff stuff. Long drives go faster when you're mentally engaged in something in addition to driving (unless there's bad weather - do not drive 12 hours in bad weather.)