A long trip is nothing more than a bunch of day trips. Some days you travel. Some days you don't. We recently returned from a 143 day trip to Alaska. Not long enough, even for the 4th time doing it.
We would have liked to have been more flexible with time but along the way there were events on certain dates that we wanted to attend so we had to plan around them. It took some research but the itinerary fell into place with 'spare' days here & there to allow for shopping, laundry, oil changes, weather delays or just liked where we were.
We are off again this week for a 50 plus day trip with events planned for almost every weekend.
In planning start with the beginning location. Plot the most distant location. Now start adding POIs, events & attractions in between but not necessarily on the direct route. Next estimate how far in a travelling day that you are comfortable with. anything over 4 hours is a long day for us. Plot those overnight AREAS give or take an hour of driving. Now you should have a very basic trip & how long needed planned without allowing time for POIs.
Lots more online research to do but from here you have the framework to add too.