I drove across Iowa... seems like it would get a bit boring after the first few hours, unless they keep really interesting bits away from the highway in order to persuade passers-by to just keep passing.
60-80 miles a day almost sounds fun!
I've heard of that one. It does sound fun, and I'm assuming it takes mostly backroads so it would be more interesting than just driving straight through on the freeway. You miss a lot of small, quaint towns when you stick to the main freeways. There are lots of small to medium towns that I never would have visited if they weren't on backroad bike routes.
There's a big mountain ride I've heard about in Colorado that sounds pretty challenging but very beautiful that's a little tempting to me.
I think probably the best one I've ever done, though, is the RAMROD (Ride Around Mt. Rainier in One Day). It's a challenging ride, but it's so incredibly beautiful up there you don't mind the torture so much.