For the time being, just gradually increase your mileage, both per ride and per week. In terms of RAGBRAI, start doing long back-to-back rides.
There's no one "right" way to do a tour. Some really like to take it slowly, stopping at each rest stop or town, having a leisurely lunch. Some like to push their pace. I've done it both ways and probably prefer a happy medium between the two. I'd at least like to be in good enough shape to ride hard any given day so, for me, that means some fairly high mileage weeks (175 plus miles) and some long back-to-back rides. Plus, I'd do some hill work depending on the terrain. If wind is a potential issue, I'd train for that, too.
If it gives you any hope, I did a week long tour my first full year of cycling. It's a doable goal and a lot of fun. Good luck!
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