Driving 800 miles in a day on a road trip is safe and doable as long as you're up to the task and have gotten plenty of sleep the night before.
I've driven 800 miles in one day on a road trip to get to my destination and by the time I got there I was really tired and ready to sleep.
Driving too long can be very tiring and exhausting so if you feel tired and exhausted during the road trip you should pull over and try to find a motel or hotel or at least sleep in your car.
Make sure to get 8 to 9 hours of sleep the night before the road trip so that you're well rested.
During the 800 mile road trip you should pull over and stop and get out and stretch your legs and walk around every 1 hour to 2 hours.
Make sure you keep your gas tank filled up when you can as well because there can be roads where there could be 50 to 100 miles between gas stations depending on where you're driving.