Most of the riders I saw were doing about 10 mi. over the speedlimit..[ie: 75 in a 65 mph zone - more or less kept them in the traffic flow] so if you take a 15 min break every 90 min. , I'd be looking at a 5-6 hour day to cover 300-400? does that sound right?
I don't think this is reasonable for someone just getting into distance riding. I wouldn't plan to average more than 50 mph with stops, so that would be 6 hours for 300 miles. Push it to 8 hours to cover 400. If you travel with others, you'll be going even slower.
If you plan to be done in 6 hours but it takes 9, you get in a hurry and make bad decisions - staying on the biek too long, getting tired, not eating...). But if you plan for 9 and it takes 7, then everyone's happy!
Don't push too far until you know what you're getting into.
All that said, I ride 200 miles between stops and stop long enough to get gas, have a snack and a drink, stretch, go to the bathroom. Takes 20 minutes usually unless I'm in a hurry, then it takes 10. If it's all interstate and traffic is light, I can average 65 MPH that way all day. Any other factors and the average goes down rapidly.