Miles per day depends on how I'm spending the night, how long I want to spend on meals, and how late in the day I want to go. Also, is it all high speed freeway, or somewhat slower highways or even local roads. And any stops for walks, sightseeing etc.
For us 450 miles per day is about max, especially if camping (which has more setup and take down time than motels). And a push one day might be balanced out with a more leisurely day nest. For a 3 mth trip to Alaska we averaged about 250 miles a day.
On other hand, some people rotate drivers and travel cross country nonstop.
When much young we drove to New England and back to Indiana. The return had one day from Conn to Youngstown OH, and another to Elkhart Indiana. The next leg was 1 day to Minneapolis, and another to North Dakota - so 4-5 days sounds reasonable for Mass to Black Hills.
It's also possible to overplan. There's value in having lots of information, and a general idea, and then settle on details as you go along. I've never followed my initial plans exactly.
