You are way ahead of where we were before our TA. We didn't have any idea we were going until 8 weeks before departure and it wasn't a sure thing yet for a few more weeks after that
Personally I would not find that gearing adequate, but others certainly have had higher. What route are you planning to take? Is it even possible to not cross the Rockies without being in the extreme southern part of the US? I didn't think so. In any case you will find LOTS of climbing on any coast to coast trip.
In the west I found 46-36-26 and 11-32 to be just adequate in the Cascades and Rockies and a littler higher than I would have liked in the worst spots in the steeper Appalachians. At the very least I would put on the smallest inner ring that fits your crankset. You can get one for about $12 so why not?
As an aside... we rode with a guy a bit on the TA. He was young and strong and was the first to the top of every climb despite running road gearing. He didn't make it through two states before he blew out his knee though.
You didn't ask about route or maps, but I will make a suggestion anyway. We found the Adventure Cycling maps to be a huge help and well worth the money. They contain much info about where to stay and where you can find all of the needed services. Post offices, stores, camping places, bike shops, motels, B&Bs, hostels, places of interest, and restaurants are all listed. The maps include info about the area, turn by turn directions, and profiles for the route in the western mountains.
I suggest taking one of their standard routes maybe the Northern Tier or the TransAmerica. Ether can be combined with the Lewis and Clark.
I will also plug my web page journal because we tried to document what we did and what did and didn't work well for us. The site is at:
http://www.crazyguyonabike.com/doc/staehling2007
If anything there raises any questions feel free to ask either here, in the journals guest book, or via email.
Have a great trip!