I plan in hours because I also plan the time for the stops. Stops can be several hours with zero miles.
In the RV, I plan 14 hours a day being awake, 8-10 hours a day from campsite to campsite. A 10 hour day might take me 450-600 miles on a travel day, fewer than 100 miles on a sightseeing day.
That's why "miles per day" doesn't work. If you are starting off on a trip with a number in your head like 300 miles a day or 500 miles a day, you don't have a plan. That's not a plan, it is at best a rule of thumb, more likely a WAG.
