I'll be taking a 5300 mile trip this year, but it will be a 31-day trip. On the "big" driving days, we're limiting ourselves to no more than about 400 miles. We have a couple of those planned. Most days are 300 to 350 miles. I've found that I can reliably figure on an average of 50MPH, counting all stops and traffic issues. So, a 6-hour to 7-hour day is a chore, but not usually too much of a challenge. Then of course there are the vast majority of days where we either stay put or just tour around the area we're exploring.
Everyone is different, aren't they?
-Speak