We limit our long days to 250 miles, most of the time it is 150-200. We sight see and take 2 lane roads whenever possible. When we travel it is for the enjoyment of the trip, not to get from A to B as fast as possible. It take us 7 days to travel the 1550 miles when we go to see our son. We used to do the 300-400 mile days, but all one sees is highway. For us that is not enjoyable. We have slowed down and now take our time to see this wonderful country, stopping often to read historical markers, look at the scenery at turnouts, etc. I have a great despise for freeways so I am always planning our trips on side roads to see new country.
Now all that being said, if you are under a time crunch of say 2-3 weeks and want to go coast to coast because you what you want to see is on the other coast then yes, you have to drive 300-400 miles a day and it is doable. I know a guy who is horns to the wind and does 600-700 miles a day pulling 5er, because for him the driving is an inconvenience, the fun is at his destination. We would never make a trip together LOL.
So you need to decide "why" you are making the trip, that will then set your daily travel goals. Good Luck and above all be safe in your travels.