Lizzie, the GS trip planner has only short limits. When I started my planning for Alaska I could only "Plan" from Marietta Ohio to Denver because it will only do so much, the I did another section from Denver to Boise ID.
It took 6 different "planned trips" to get me to Alaska and back to Ohio.
To direct the routes you must use "Waypoints" and the system will only accept 15 waypoints per route. I needed more because I modified my route to stay off Interstates, and added stops for pictures, exercise trails, and goof off points.
GGeorge