This may seem overly simplistic, but in my opinion, the best way to train for a weeklong tour like RAGBRAI is to ride lots. For me, that means riding at least 4 days a week, with back-to-back long rides on both Saturday and Sunday. I'm doing a tour in mid-September in Tennessee and I've been averaging (with a few exceptions) between 150 and 200 miles a week since late April. I've had a couple of light weeks lately, but I'll make one final push these next couple of weeks. After Labor Day, I'll likely start taking it easy again
If there will be any hilly days, then you also have to incorporate hilly rides (or windy rides if you don't have hills) into your training. Finally, if you're not used to riding in a group, then you might want to work on your group riding skills. What kind of a "team" does your friend belond to and what are their expectations, especially in terms of speed? You might ask your friend or her teammates how they train.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher