RoadTime has a good point. The body and mind need time to adapt.
I remembered the "pain" when I first drove 800 miles. It wasn't physical pain, it's more like the mental pain. I fought trying to keep awake and alert to deliver the load.
Now, my body and mind are working in concert, adapted that is. I have developed the habit of preplanning the rest before the next assignment. During the trip, my body and mind will give me the signs when to stop for the short rest, sleep, breaks, etc.
As months and years passed, I noticed my tolerance to drive longer distance have improved.
Everyone tolerance is difference, but here's my patterns.
Up to 800 miles- 6 months.
800-2000 miles- 3 years.
2000 miles and up-I'm still working on it. Lol...
Please do it safely.
EDIT: Oh by the way, most of the miles mentioned above are out and back miles.