I have been riding for about 10 years or so on a Cannondale SR500 and am doing my first tour this spring - van supported Southern Tier. I am looking to buy a new bike for the tour but also hoping to find one that will work well as my everyday bike.
The only recommendation I can give you is the least helpful: Ride Lots. Try to get up to a 50 mile ride once or twice a week. You might have to go to spin classes if the winter in Helena is not conducive to riding outside. 3-5 rides a week, increasing mileage about 10% a week, has been the standard prescription for years; it's a good 'script. Start now, if you're going to be ready to ride next spring.
Sometimes on longer event rides, I do hook up with a group of riders and would like to be able to have a bike that would give me a reasonable chance to keep up with riders of similar ability.
I'm afraid you've got that. Those "riders of similar ability" have better endurance, or ability to ride faster than you, because they've trained longer and/or harder. They actually have more ability than you do at this point.
Your bike is 90% of the best you can get. A fitting from a professional who knows what he's doing would be the best $200 you'd ever spend on a bike. Swap the stem? Adjust seat height? Tweak the seat setback? The fitter can help you with that. At 54, you might well need a higher stem, especially for long days in the saddle. You don't really need a touring bike for a van supported tour, although the lower gearing might be welcome when you start climbing mountains! (Note I'm saying that with two of my three bikes being touring bikes, and those are the ones I've taken to all my supported tours!)
Have you considered doing a one-week van supported trip next year, and hold off on the Southern Tier for another year? That would give you a chance to get into shape for the longer trip.