We did 4900 miles in 14 days this past August. Year before 4300 miles in 20 days. If you travel fairly close to home I'd suggest a loop that will let you sight see all along the route. If you plan a farther route I'd suggest 1 way balls to the wall and the other way a slow sight seeing ride.
One suggestion from Indiana might be a fast ride along I-70 to the Rocky Mountains. Zig zag your way through the Rockies (MUCH to see), cross over from Glacier Park to Waterton Park in Canada and ride the Canadian Rockies then head home along CA1 and loop around the Great Lakes.
Don't know that I'd try the pacific Coast Hwy from IN with 3 weeks but it's doable. That's be one helluva loop that I could help you with.
East of the Mississippi for 3 weeks, cruise around the Great Lakes, cross over in Canada, work your way over to Maine and visit New England. I lived in KY most of my first 47 years and in Colorado since and have found that western US riding is SO much more superior. Open roads, open spaces, less people and traffic, way more grandeur.