The TransAm is the original route and probably a couple of thousand ride across it. Since you are not overly experienced, you should avoid doing Route 66 to save a few miles as parts of it are a bit heavy traffic and/or no-shoulders.
Are you mostly camping, staying in motels, eating out, cooking your own food, traveling solo or with other buddies, etc.? These answers can help with route suggestions. Believe it or not, 74 is not uncommon when it comes to bike touring. I regularly do tours with people in their mid to late 70s. As long as you are in fair shape (but great shape helps!) and more importantly know your limits, you will do fine.